NullableWalker.cs

// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.

#if DEBUG
// See comment in DefiniteAssignment.
#define REFERENCE_STATE
#endif

using System;
using System.Collections.Generic;
using System.Collections.Immutable;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using Microsoft.CodeAnalysis.CSharp.Symbols;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.PooledObjects;
using Roslyn.Utilities;

namespace Microsoft.CodeAnalysis.CSharp
{
    /// <summary>
    /// Nullability flow analysis.
    /// </summary>
    [DebuggerDisplay("{GetDebuggerDisplay(), nq}")]
    internal sealed partial class NullableWalker : LocalDataFlowPass<NullableWalker.LocalState>
    {
        /// <summary>
        /// Used to copy variable slots and types from the NullableWalker for the containing method
        /// or lambda to the NullableWalker created for a nested lambda or local function.
        /// </summary>
        internal sealed class VariableState
        {
            // Consider referencing the collections directly from the original NullableWalker
            // rather than copying the collections. (Items are added to the collections
            // but never replaced so the collections are lazily populated but otherwise immutable.)
            internal readonly ImmutableDictionary<VariableIdentifier, int> VariableSlot;
            internal readonly ImmutableArray<VariableIdentifier> VariableBySlot;
            internal readonly ImmutableDictionary<Symbol, TypeWithAnnotations> VariableTypes;

            internal VariableState(
                ImmutableDictionary<VariableIdentifier, int> variableSlot,
                ImmutableArray<VariableIdentifier> variableBySlot,
                ImmutableDictionary<Symbol, TypeWithAnnotations> variableTypes)
            {
                VariableSlot = variableSlot;
                VariableBySlot = variableBySlot;
                VariableTypes = variableTypes;
            }
        }

        /// <summary>
        /// Represents the result of visiting an expression.
        /// Contains a result type which tells us whether the expression may be null,
        /// and an l-value type which tells us whether we can assign null to the expression.
        /// </summary>
        [DebuggerDisplay("{GetDebuggerDisplay(), nq}")]
        private readonly struct VisitResult
        {
            public readonly TypeWithState RValueType;
            public readonly TypeWithAnnotations LValueType;

            public VisitResult(TypeWithState rValueType, TypeWithAnnotations lValueType)
            {
                RValueType = rValueType;
                LValueType = lValueType;
            }

            private string GetDebuggerDisplay() =>
                $"RValueType={RValueType.GetDebuggerDisplay()}, LValueType={LValueType.GetDebuggerDisplay()}";
        }

        /// <summary>
        /// The inferred type at the point of declaration of var locals and parameters.
        /// </summary>
        private readonly PooledDictionary<Symbol, TypeWithAnnotations> _variableTypes = PooledDictionary<Symbol, TypeWithAnnotations>.GetInstance();

        private readonly Binder _binder;

        /// <summary>
        /// Conversions with nullability and unknown matching any.
        /// </summary>
        private readonly Conversions _conversions;

        /// <summary>
        /// Use the return type and nullability from _methodSignatureOpt to calculate return
        /// expression conversions. If false, the signature of _member is used instead.
        /// </summary>
        private readonly bool _useMethodSignatureReturnType;

        /// <summary>
        /// Use the the parameter types and nullability from _methodSignatureOpt for initial
        /// parameter state. If false, the signature of _member is used instead.
        /// </summary>
        private readonly bool _useMethodSignatureParameterTypes;

        /// <summary>
        /// Method signature used for return type or parameter types. Distinct from _member
        /// signature when _member is a lambda and type is inferred from MethodTypeInferrer.
        /// </summary>
        private readonly MethodSymbol _methodSignatureOpt;

        /// <summary>
        /// Return statements and the result types from analyzing the returned expressions. Used when inferring lambda return type in MethodTypeInferrer.
        /// </summary>
        private readonly ArrayBuilder<(BoundReturnStatement, TypeWithAnnotations)> _returnTypesOpt;

        /// <summary>
        /// An optional callback for callers to receive notification of the inferred type and nullability
        /// of each expression in the method. Since the walker may require multiple passes, the callback
        /// may be invoked multiple times for a single expression, potentially with different nullability
        /// each time. The last call for each expression will include the final inferred type and nullability.
        /// </summary>
        private readonly Action<BoundExpression, TypeWithAnnotations> _callbackOpt;

        /// <summary>
        /// Invalid type, used only to catch Visit methods that do not set
        /// _result.Type. See VisitExpressionWithoutStackGuard.
        /// </summary>
        private static readonly TypeWithState _invalidType = new TypeWithState(ErrorTypeSymbol.UnknownResultType, NullableFlowState.NotNull);

        /// <summary>
        /// The result and l-value type of the last visited expression.
        /// </summary>
        private VisitResult _visitResult;

        /// <summary>
        /// The visit result of the receiver for the current conditional access.
        ///
        /// For example: A conditional invocation uses a placeholder as a receiver. By storing the
        /// visit result from the actual receiver ahead of time, we can give this placeholder a correct result.
        /// </summary>
        private VisitResult _currentConditionalReceiverVisitResult;

        /// <summary>
        /// The result type represents the state of the last visited expression.
        /// </summary>
        private TypeWithState ResultType
        {
            get => _visitResult.RValueType;
            set
            {
                SetResult(rvalueType: value, lvalueType: value.ToTypeWithAnnotations());
            }
        }

        /// <summary>
        /// Force the inference of the LValueResultType from ResultType.
        /// </summary>
        private void UseRvalueOnly()
        {
            ResultType = ResultType;
        }

        private TypeWithAnnotations LvalueResultType
        {
            get => _visitResult.LValueType;
            set
            {
                SetResult(rvalueType: value.ToTypeWithState(), lvalueType: value);
            }
        }

        /// <summary>
        /// Force the inference of the ResultType from LValueResultType.
        /// </summary>
        private void UseLvalueOnly()
        {
            LvalueResultType = LvalueResultType;
        }

        private void SetResult(TypeWithState rvalueType, TypeWithAnnotations lvalueType)
        {
            _visitResult = new VisitResult(rvalueType, lvalueType);
        }

        /// <summary>
        /// Instances being constructed.
        /// </summary>
        private PooledDictionary<BoundExpression, ObjectCreationPlaceholderLocal> _placeholderLocalsOpt;

        /// <summary>
        /// For methods with annotations, we'll need to visit the arguments twice.
        /// Once for diagnostics and once for result state (but disabling diagnostics).
        /// </summary>
        private bool _disableDiagnostics = false;

        /// <summary>
        /// Used to allow <see cref="MakeSlot(BoundExpression)"/> to substitute the correct slot for a <see cref="BoundConditionalReceiver"/> when
        /// it's encountered.
        /// </summary>
        private int _lastConditionalAccessSlot = -1;

        protected override void Free()
        {
            _variableTypes.Free();
            _placeholderLocalsOpt?.Free();
            base.Free();
        }

        private NullableWalker(
            CSharpCompilation compilation,
            MethodSymbol method,
            bool useMethodSignatureReturnType,
            bool useMethodSignatureParameterTypes,
            MethodSymbol methodSignatureOpt,
            BoundNode node,
            ArrayBuilder<(BoundReturnStatement, TypeWithAnnotations)> returnTypesOpt,
            VariableState initialState,
            Action<BoundExpression, TypeWithAnnotations> callbackOpt)
            : base(compilation, method, node, new EmptyStructTypeCache(compilation, dev12CompilerCompatibility: false), trackUnassignments: true)
        {
            _callbackOpt = callbackOpt;
            _binder = compilation.GetBinderFactory(node.SyntaxTree).GetBinder(node.Syntax);
            Debug.Assert(!_binder.Conversions.IncludeNullability);
            _conversions = (Conversions)_binder.Conversions.WithNullability(true);
            _useMethodSignatureReturnType = (object)methodSignatureOpt != null && useMethodSignatureReturnType;
            _useMethodSignatureParameterTypes = (object)methodSignatureOpt != null && useMethodSignatureParameterTypes;
            _methodSignatureOpt = methodSignatureOpt;
            _returnTypesOpt = returnTypesOpt;
            if (initialState != null)
            {
                var variableBySlot = initialState.VariableBySlot;
                nextVariableSlot = variableBySlot.Length;
                foreach (var (variable, slot) in initialState.VariableSlot)
                {
                    Debug.Assert(slot < nextVariableSlot);
                    _variableSlot.Add(variable, slot);
                }
                this.variableBySlot = variableBySlot.ToArray();
                foreach (var pair in initialState.VariableTypes)
                {
                    _variableTypes.Add(pair.Key, pair.Value);
                }
            }
        }

        public string GetDebuggerDisplay()
        {
            if (this.IsConditionalState)
            {
                return $"{{{GetType().Name} WhenTrue:{Dump(StateWhenTrue)} WhenFalse:{Dump(StateWhenFalse)}{"}"}";
            }
            else
            {
                return $"{{{GetType().Name} {Dump(State)}{"}"}";
            }
        }

        // For purpose of nullability analysis, awaits create pending branches, so async usings and foreachs do too
        public sealed override bool AwaitUsingAndForeachAddsPendingBranch => true;

        protected override bool ConvertInsufficientExecutionStackExceptionToCancelledByStackGuardException()
        {
            return true;
        }

        protected override ImmutableArray<PendingBranch> Scan(ref bool badRegion)
        {
            if (_returnTypesOpt != null)
            {
                _returnTypesOpt.Clear();
            }
            this.Diagnostics.Clear();
            ParameterSymbol methodThisParameter = MethodThisParameter;
            this.State = TopState();                   // entry point is reachable
            this.regionPlace = RegionPlace.Before;
            EnterParameters();                               // with parameters assigned
            if (!(methodThisParameter is null))
            {
                EnterParameter(methodThisParameter, methodThisParameter.TypeWithAnnotations);
            }

            ImmutableArray<PendingBranch> pendingReturns = base.Scan(ref badRegion);
            return pendingReturns;
        }

        internal static void Analyze(
            CSharpCompilation compilation,
            MethodSymbol method,
            BoundNode node,
            DiagnosticBag diagnostics,
            Action<BoundExpression, TypeWithAnnotations> callbackOpt = null)
        {
            if (method.IsImplicitlyDeclared && (!method.IsImplicitConstructor || method.ContainingType.IsImplicitlyDeclared))
            {
                return;
            }
            Analyze(compilation, method, node, diagnostics, useMethodSignatureReturnType: false, useMethodSignatureParameterTypes: false, methodSignatureOpt: null, returnTypes: null, initialState: null, callbackOpt);
        }

        internal static void AnalyzeIfNeeded(
            CSharpCompilation compilation,
            BoundAttribute attribute,
            DiagnosticBag diagnostics)
        {
            if (compilation.LanguageVersion < MessageID.IDS_FeatureNullableReferenceTypes.RequiredVersion())
            {
                return;
            }

            Analyze(compilation, null, attribute, diagnostics, useMethodSignatureReturnType: false, useMethodSignatureParameterTypes: false, methodSignatureOpt: null, returnTypes: null, initialState: null, callbackOpt: null);
        }

        internal static void Analyze(
            CSharpCompilation compilation,
            BoundLambda lambda,
            DiagnosticBag diagnostics,
            MethodSymbol delegateInvokeMethod,
            ArrayBuilder<(BoundReturnStatement, TypeWithAnnotations)> returnTypes,
            VariableState initialState)
        {
            Analyze(
                compilation,
                lambda.Symbol,
                lambda.Body,
                diagnostics,
                useMethodSignatureReturnType: true,
                useMethodSignatureParameterTypes: !lambda.UnboundLambda.HasExplicitlyTypedParameterList,
                methodSignatureOpt: delegateInvokeMethod,
                returnTypes, initialState,
                callbackOpt: null);
        }

        private static void Analyze(
            CSharpCompilation compilation,
            MethodSymbol method,
            BoundNode node,
            DiagnosticBag diagnostics,
            bool useMethodSignatureReturnType,
            bool useMethodSignatureParameterTypes,
            MethodSymbol methodSignatureOpt,
            ArrayBuilder<(BoundReturnStatement, TypeWithAnnotations)> returnTypes,
            VariableState initialState,
            Action<BoundExpression, TypeWithAnnotations> callbackOpt)
        {
            Debug.Assert(diagnostics != null);
            var walker = new NullableWalker(compilation, method, useMethodSignatureReturnType, useMethodSignatureParameterTypes, methodSignatureOpt, node, returnTypes, initialState, callbackOpt);
            try
            {
                bool badRegion = false;
                ImmutableArray<PendingBranch> returns = walker.Analyze(ref badRegion);
                diagnostics.AddRange(walker.Diagnostics);
                Debug.Assert(!badRegion);
            }
            catch (BoundTreeVisitor.CancelledByStackGuardException ex) when (diagnostics != null)
            {
                ex.AddAnError(diagnostics);
            }
            finally
            {
                walker.Free();
            }
        }

        protected override void Normalize(ref LocalState state)
        {
            if (!state.Reachable)
                return;

            int oldNext = state.Capacity;
            state.EnsureCapacity(nextVariableSlot);
            Populate(ref state, oldNext);
        }

        private void Populate(ref LocalState state, int start)
        {
            int capacity = state.Capacity;
            for (int slot = start; slot < capacity; slot++)
            {
                state[slot] = GetDefaultState(ref state, slot);
            }
        }

        private NullableFlowState GetDefaultState(ref LocalState state, int slot)
        {
            if (!state.Reachable)
                return NullableFlowState.NotNull;

            if (slot == 0)
                return NullableFlowState.MaybeNull;

            var variable = variableBySlot[slot];
            var symbol = variable.Symbol;

            switch (symbol.Kind)
            {
                case SymbolKind.Local:
                    // Locals are considered not null before they are definitely assigned
                    return NullableFlowState.NotNull;
                case SymbolKind.Parameter:
                    {
                        var parameter = (ParameterSymbol)symbol;
                        if (parameter.RefKind == RefKind.Out)
                        {
                            return NullableFlowState.NotNull;
                        }

                        if (!_variableTypes.TryGetValue(parameter, out TypeWithAnnotations parameterType))
                        {
                            parameterType = parameter.TypeWithAnnotations;
                        }

                        return parameterType.ToTypeWithState().State;
                    }
                case SymbolKind.Field:
                case SymbolKind.Property:
                case SymbolKind.Event:
                    return symbol.GetTypeOrReturnType().ToTypeWithState().State;
                default:
                    throw ExceptionUtilities.UnexpectedValue(symbol.Kind);
            }
        }

        protected override bool TryGetReceiverAndMember(BoundExpression expr, out BoundExpression receiver, out Symbol member)
        {
            receiver = null;
            member = null;

            switch (expr.Kind)
            {
                case BoundKind.FieldAccess:
                    {
                        var fieldAccess = (BoundFieldAccess)expr;
                        var fieldSymbol = fieldAccess.FieldSymbol;
                        member = fieldSymbol;
                        if (fieldSymbol.IsFixedSizeBuffer)
                        {
                            return false;
                        }
                        if (fieldSymbol.IsStatic)
                        {
                            return true;
                        }
                        receiver = fieldAccess.ReceiverOpt;
                        break;
                    }
                case BoundKind.EventAccess:
                    {
                        var eventAccess = (BoundEventAccess)expr;
                        var eventSymbol = eventAccess.EventSymbol;
                        // https://github.com/dotnet/roslyn/issues/29901 Use AssociatedField for field-like events?
                        member = eventSymbol;
                        if (eventSymbol.IsStatic)
                        {
                            return true;
                        }
                        receiver = eventAccess.ReceiverOpt;
                        break;
                    }
                case BoundKind.PropertyAccess:
                    {
                        var propAccess = (BoundPropertyAccess)expr;
                        var propSymbol = propAccess.PropertySymbol;
                        member = GetBackingFieldIfStructProperty(propSymbol);
                        if (member is null)
                        {
                            return false;
                        }
                        if (propSymbol.IsStatic)
                        {
                            return true;
                        }
                        receiver = propAccess.ReceiverOpt;
                        break;
                    }
            }

            Debug.Assert(member?.IsStatic != true);

            return (object)member != null &&
                (object)receiver != null &&
                receiver.Kind != BoundKind.TypeExpression &&
                (object)receiver.Type != null;
        }

        // https://github.com/dotnet/roslyn/issues/29619 Use backing field for struct property
        // for now, to avoid cycles if the struct type contains a property of the struct type.
        // Remove this and populate struct members lazily to match classes.
        private Symbol GetBackingFieldIfStructProperty(Symbol symbol)
        {
            if (symbol.Kind == SymbolKind.Property && !symbol.ContainingType.IsNullableType())
            {
                var property = (PropertySymbol)symbol;
                var containingType = property.ContainingType;
                if (containingType.TypeKind == TypeKind.Struct)
                {
                    // https://github.com/dotnet/roslyn/issues/29619 Relying on field name
                    // will not work for properties declared in other languages.
                    var fieldName = GeneratedNames.MakeBackingFieldName(property.Name);
                    return _emptyStructTypeCache.GetStructFields(containingType, includeStatic: symbol.IsStatic).FirstOrDefault(f => f.Name == fieldName);
                }
            }
            return symbol;
        }

        // https://github.com/dotnet/roslyn/issues/29619 Temporary, until we're using
        // properties on structs directly.
        protected override int GetOrCreateSlot(Symbol symbol, int containingSlot = 0)
        {
            symbol = GetBackingFieldIfStructProperty(symbol);
            if (symbol is null)
            {
                return -1;
            }
            return base.GetOrCreateSlot(symbol, containingSlot);
        }

        protected override int MakeSlot(BoundExpression node)
        {
            switch (node.Kind)
            {
                case BoundKind.ThisReference:
                case BoundKind.BaseReference:
                    {
                        var method = getTopLevelMethod(_symbol as MethodSymbol);
                        var thisParameter = method?.ThisParameter;
                        return (object)thisParameter != null ? GetOrCreateSlot(thisParameter) : -1;
                    }
                case BoundKind.Conversion:
                    {
                        var conv = (BoundConversion)node;
                        switch (conv.Conversion.Kind)
                        {
                            case ConversionKind.ExplicitNullable:
                                {
                                    var operand = conv.Operand;
                                    var operandType = operand.Type;
                                    var convertedType = conv.Type;
                                    if (AreNullableAndUnderlyingTypes(operandType, convertedType, out _))
                                    {
                                        // Explicit conversion of Nullable<T> to T is equivalent to Nullable<T>.Value.
                                        // For instance, in the following, when evaluating `((A)a).B` we need to recognize
                                        // the nullability of `(A)a` (not nullable) and the slot (the slot for `a.Value`).
                                        //   struct A { B? B; }
                                        //   struct B { }
                                        //   if (a?.B != null) _ = ((A)a).B.Value; // no warning
                                        int containingSlot = MakeSlot(operand);
                                        return containingSlot < 0 ? -1 : GetNullableOfTValueSlot(operandType, containingSlot, out _);
                                    }
                                    else if (AreNullableAndUnderlyingTypes(convertedType, operandType, out _))
                                    {
                                        // Explicit conversion of T to Nullable<T> is equivalent to new Nullable<T>(t).
                                        return getPlaceholderSlot(node);
                                    }
                                }
                                break;
                            case ConversionKind.ImplicitNullable:
                                // Implicit conversion of T to Nullable<T> is equivalent to new Nullable<T>(t).
                                if (AreNullableAndUnderlyingTypes(conv.Type, conv.Operand.Type, out _))
                                {
                                    return getPlaceholderSlot(node);
                                }
                                break;
                            case ConversionKind.Identity:
                            case ConversionKind.ImplicitReference:
                            case ConversionKind.ExplicitReference:
                            case ConversionKind.ImplicitTupleLiteral:
                            case ConversionKind.ExplicitTupleLiteral:
                            case ConversionKind.ImplicitTuple:
                            case ConversionKind.ExplicitTuple:
                            case ConversionKind.Boxing:
                            case ConversionKind.Unboxing:
                                if (isSupportedConversion(conv.Conversion, conv.Operand))
                                {
                                    // No need to create a slot for the boxed value (in the Boxing case) since assignment already
                                    // clones slots and there is not another scenario where creating a slot is observable.
                                    return MakeSlot(conv.Operand);
                                }
                                break;
                        }
                    }
                    break;
                case BoundKind.DefaultExpression:
                case BoundKind.ObjectCreationExpression:
                case BoundKind.DynamicObjectCreationExpression:
                case BoundKind.AnonymousObjectCreationExpression:
                case BoundKind.NewT:
                case BoundKind.TupleLiteral:
                case BoundKind.ConvertedTupleLiteral:
                    return getPlaceholderSlot(node);
                case BoundKind.ConditionalReceiver:
                    {
                        return _lastConditionalAccessSlot;
                    }
                default:
                    // If there was a placeholder local for this node, we should
                    // use the placeholder for the slot. See other cases above.
                    Debug.Assert(_placeholderLocalsOpt?.TryGetValue(node, out _) != true);
                    return base.MakeSlot(node);
            }

            return -1;

            int getPlaceholderSlot(BoundExpression expr)
            {
                if (_placeholderLocalsOpt != null && _placeholderLocalsOpt.TryGetValue(expr, out ObjectCreationPlaceholderLocal placeholder))
                {
                    return GetOrCreateSlot(placeholder);
                }
                return -1;
            }

            static MethodSymbol getTopLevelMethod(MethodSymbol method)
            {
                while ((object)method != null)
                {
                    var container = method.ContainingSymbol;
                    if (container.Kind == SymbolKind.NamedType)
                    {
                        return method;
                    }
                    method = container as MethodSymbol;
                }
                return null;
            }

            // Returns true if the nullable state from the operand of the conversion
            // can be used as is, and we can create a slot from the conversion.
            static bool isSupportedConversion(Conversion conversion, BoundExpression operandOpt)
            {
                // https://github.com/dotnet/roslyn/issues/32599: Allow implicit and explicit
                // conversions where the nullable state of the operand remains valid.
                switch (conversion.Kind)
                {
                    case ConversionKind.Identity:
                    case ConversionKind.DefaultOrNullLiteral:
                    case ConversionKind.ImplicitReference:
                    case ConversionKind.ExplicitReference:
                    case ConversionKind.Boxing:
                    case ConversionKind.Unboxing:
                        return true;
                    case ConversionKind.ImplicitTupleLiteral:
                    case ConversionKind.ExplicitTupleLiteral:
                        switch (operandOpt?.Kind)
                        {
                            case BoundKind.Conversion:
                                {
                                    var operandConversion = (BoundConversion)operandOpt;
                                    return isSupportedConversion(operandConversion.Conversion, operandConversion.Operand);
                                }
                            case BoundKind.ConvertedTupleLiteral:
                                {
                                    var arguments = ((BoundConvertedTupleLiteral)operandOpt).Arguments;
                                    var conversions = conversion.UnderlyingConversions;
                                    for (int i = 0; i < arguments.Length; i++)
                                    {
                                        // https://github.com/dotnet/roslyn/issues/32600: Copy nullable
                                        // state of tuple elements independently.
                                        if (!isSupportedConversion(conversions[i], (arguments[i] as BoundConversion)?.Operand))
                                        {
                                            return false;
                                        }
                                    }
                                    return true;
                                }
                            default:
                                return false;
                        }
                    case ConversionKind.ImplicitTuple:
                    case ConversionKind.ExplicitTuple:
                        // https://github.com/dotnet/roslyn/issues/32600: Copy nullable
                        // state of tuple elements independently.
                        return conversion.UnderlyingConversions.All(c => isSupportedConversion(c, null));
                    default:
                        return false;
                }
            }
        }

        protected override void VisitRvalue(BoundExpression node)
        {
            Visit(node);
            VisitRvalueEpilogue();
        }

        /// <summary>
        /// The contents of this method, particularly <see cref="UseRvalueOnly"/>, are problematic when 
        /// inlined. The methods themselves are small but they end up allocating significantly larger 
        /// frames due to the use of biggish value types within them. The <see cref="VisitRvalue"/> method
        /// is used on a hot path for fluent calls and this size change is enough that it causes us
        /// to exceed our thresholds in EndToEndTests.OverflowOnFluentCall.
        /// </summary>
        [MethodImpl(MethodImplOptions.NoInlining)]
        private void VisitRvalueEpilogue()
        {
            Unsplit();
            UseRvalueOnly(); // drop lvalue part
        }

        private TypeWithState VisitRvalueWithState(BoundExpression node)
        {
            VisitRvalue(node);
            return ResultType;
        }

        private TypeWithAnnotations VisitLvalueWithAnnotations(BoundExpression node)
        {
            Visit(node);
            Unsplit();
            return LvalueResultType;
        }

        private static object GetTypeAsDiagnosticArgument(TypeSymbol typeOpt)
        {
            return typeOpt ?? (object)"<null>";
        }

        private enum AssignmentKind
        {
            Assignment,
            Return,
            Argument
        }

        /// <summary>
        /// Reports top-level nullability problem in assignment.
        /// </summary>
        private bool ReportNullableAssignmentIfNecessary(
            BoundExpression value,
            TypeWithAnnotations targetType,
            TypeWithState valueType,
            bool useLegacyWarnings,
            AssignmentKind assignmentKind = AssignmentKind.Assignment,
            Symbol target = null,
            Conversion conversion = default)
        {
            Debug.Assert((object)target != null || assignmentKind != AssignmentKind.Argument);

            if (value == null ||
                !targetType.HasType ||
                targetType.Type.IsValueType ||
                targetType.CanBeAssignedNull ||
                valueType.IsNotNull)
            {
                return false;
            }

            var unwrappedValue = SkipReferenceConversions(value);
            if (unwrappedValue.IsSuppressed)
            {
                return false;
            }

            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            if (RequiresSafetyWarningWhenNullIntroduced(targetType.Type))
            {
                if (conversion.Kind == ConversionKind.UnsetConversionKind)
                    conversion = this._conversions.ClassifyImplicitConversionFromType(valueType.Type, targetType.Type, ref useSiteDiagnostics);

                if (conversion.IsImplicit && !conversion.IsDynamic)
                {
                    // For type parameters that cannot be annotated, the analysis must report those
                    // places where null values first sneak in, like `default`, `null`, and `GetFirstOrDefault`,
                    // as a safety diagnostic.  This is NOT one of those places.
                    return false;
                }

                useLegacyWarnings = false;
            }

            if (reportNullLiteralAssignmentIfNecessary(value))
            {
                return true;
            }

            if (assignmentKind == AssignmentKind.Argument)
            {
                ReportSafetyDiagnostic(ErrorCode.WRN_NullReferenceArgument, value.Syntax,
                    new FormattedSymbol(target, SymbolDisplayFormat.ShortFormat),
                    new FormattedSymbol(target.ContainingSymbol, SymbolDisplayFormat.MinimallyQualifiedFormat));
            }
            else if (useLegacyWarnings)
            {
                ReportNonSafetyDiagnostic(value.Syntax);
            }
            else
            {
                ReportSafetyDiagnostic(assignmentKind == AssignmentKind.Return ? ErrorCode.WRN_NullReferenceReturn : ErrorCode.WRN_NullReferenceAssignment, value.Syntax);
            }

            return true;

            // Report warning converting null literal to non-nullable reference type.
            // target (e.g.: `object x = null;` or calling `void F(object y)` with `F(null)`).
            bool reportNullLiteralAssignmentIfNecessary(BoundExpression expr)
            {
                if (expr.ConstantValue?.IsNull != true)
                {
                    return false;
                }

                // For type parameters that cannot be annotated, the analysis must report those
                // places where null values first sneak in, like `default`, `null`, and `GetFirstOrDefault`,
                // as a safety diagnostic.  This is one of those places.
                if (useLegacyWarnings && !RequiresSafetyWarningWhenNullIntroduced(expr.Type))
                {
                    ReportNonSafetyDiagnostic(expr.Syntax);
                }
                else
                {
                    ReportSafetyDiagnostic(assignmentKind == AssignmentKind.Return ? ErrorCode.WRN_NullReferenceReturn : ErrorCode.WRN_NullAsNonNullable, expr.Syntax);
                }
                return true;
            }
        }

        private static bool IsDefaultValue(BoundExpression expr)
        {
            switch (expr.Kind)
            {
                case BoundKind.Conversion:
                    {
                        var conversion = (BoundConversion)expr;
                        return conversion.Conversion.Kind == ConversionKind.DefaultOrNullLiteral &&
                            IsDefaultValue(conversion.Operand);
                    }
                case BoundKind.DefaultExpression:
                    return true;
                default:
                    return false;
            }
        }

        // Maybe this method can be replaced by VisitOptionalImplicitConversion or ApplyConversion
        private void ReportAssignmentWarnings(
            BoundExpression value,
            TypeWithAnnotations targetType,
            TypeWithState valueType,
            bool useLegacyWarnings)
        {
            Debug.Assert(value != null);

            if (this.State.Reachable)
            {
                if (!targetType.HasType || valueType.HasNullType)
                {
                    return;
                }

                // Report top-level nullability issues
                ReportNullableAssignmentIfNecessary(value, targetType, valueType, useLegacyWarnings, AssignmentKind.Assignment);

                // Report nested nullability issues
                var sourceType = valueType.Type;
                var destinationType = targetType.Type;
                if ((object)sourceType != null && IsNullabilityMismatch(destinationType, sourceType))
                {
                    ReportNullabilityMismatchInAssignment(value.Syntax, sourceType, destinationType);
                }
            }
        }

        private void ReportNullabilityMismatchInAssignment(SyntaxNode syntaxNode, object sourceType, object destinationType)
        {
            ReportSafetyDiagnostic(ErrorCode.WRN_NullabilityMismatchInAssignment, syntaxNode, sourceType, destinationType);
        }

        /// <summary>
        /// Update tracked value on assignment.
        /// </summary>
        private void TrackNullableStateForAssignment(
            BoundExpression value,
            TypeWithAnnotations targetType,
            int targetSlot,
            TypeWithState valueType,
            int valueSlot = -1)
        {
            Debug.Assert(value != null);
            Debug.Assert(!IsConditionalState);

            if (this.State.Reachable)
            {
                if (!targetType.HasType)
                {
                    return;
                }

                if (targetSlot <= 0 || targetSlot == valueSlot)
                {
                    return;
                }

                if (targetSlot >= this.State.Capacity) Normalize(ref this.State);

                var newState = valueType.State;
                SetStateAndTrackForFinally(ref this.State, targetSlot, newState);
                InheritDefaultState(targetSlot);

                // https://github.com/dotnet/roslyn/issues/33428: Can the areEquivalentTypes check be removed
                // if InheritNullableStateOfMember asserts the member is valid for target and value?
                if (areEquivalentTypes(targetType, valueType))
                {
                    // https://github.com/dotnet/roslyn/issues/31395: We should copy all tracked state from `value` regardless of
                    // BoundNode type but we'll need to handle cycles (see NullableReferenceTypesTests.Members_FieldCycle_07).
                    // For now, we copy a limited set of BoundNode types that shouldn't contain cycles.
                    if (((targetType.Type.IsReferenceType || targetType.TypeKind == TypeKind.TypeParameter) && (isSupportedReferenceTypeValue(value) || targetType.Type.IsAnonymousType)) ||
                        targetType.IsNullableType())
                    {
                        // Nullable<T> is handled here rather than in InheritNullableStateOfTrackableStruct since that
                        // method only clones auto-properties (see https://github.com/dotnet/roslyn/issues/29619).
                        // When that issue is fixed, Nullable<T> should be handled there instead.
                        if (valueSlot > 0)
                        {
                            InheritNullableStateOfTrackableType(targetSlot, valueSlot, skipSlot: targetSlot);
                        }
                    }
                    else if (EmptyStructTypeCache.IsTrackableStructType(targetType.Type))
                    {
                        InheritNullableStateOfTrackableStruct(targetType.Type, targetSlot, valueSlot, isDefaultValue: IsDefaultValue(value), skipSlot: targetSlot);
                    }
                }
            }

            static bool areEquivalentTypes(TypeWithAnnotations target, TypeWithState assignedValue) =>
                target.Type.Equals(assignedValue.Type, TypeCompareKind.AllIgnoreOptions);

            // https://github.com/dotnet/roslyn/issues/31395: See comment above.
            static bool isSupportedReferenceTypeValue(BoundExpression value)
            {
                switch (value.Kind)
                {
                    case BoundKind.Conversion:
                        return isSupportedReferenceTypeValue(((BoundConversion)value).Operand);
                    case BoundKind.ObjectCreationExpression:
                    case BoundKind.AnonymousObjectCreationExpression:
                    case BoundKind.DynamicObjectCreationExpression:
                    case BoundKind.NewT:
                        return true;
                    default:
                        return false;
                }
            }
        }

        private void ReportNonSafetyDiagnostic(SyntaxNode syntax)
        {
            ReportNonSafetyDiagnostic(ErrorCode.WRN_ConvertingNullableToNonNullable, syntax);
        }

        private void ReportNonSafetyDiagnostic(ErrorCode errorCode, SyntaxNode syntax)
        {
            // All warnings should be in the `#pragma warning ... nullable` set.
            Debug.Assert(!ErrorFacts.NullableFlowAnalysisSafetyWarnings.Contains(MessageProvider.Instance.GetIdForErrorCode((int)errorCode)));
            Debug.Assert(ErrorFacts.NullableFlowAnalysisNonSafetyWarnings.Contains(MessageProvider.Instance.GetIdForErrorCode((int)errorCode)));
#pragma warning disable CS0618
            ReportDiagnostic(errorCode, syntax);
#pragma warning restore CS0618
        }

        private void ReportSafetyDiagnostic(ErrorCode errorCode, SyntaxNode syntaxNode, params object[] arguments)
        {
            // All warnings should be in the `#pragma warning ... nullable` set.
            Debug.Assert(ErrorFacts.NullableFlowAnalysisSafetyWarnings.Contains(MessageProvider.Instance.GetIdForErrorCode((int)errorCode)));
            Debug.Assert(!ErrorFacts.NullableFlowAnalysisNonSafetyWarnings.Contains(MessageProvider.Instance.GetIdForErrorCode((int)errorCode)));
#pragma warning disable CS0618
            ReportDiagnostic(errorCode, syntaxNode, arguments);
#pragma warning restore CS0618
        }

        [Obsolete("Use ReportSafetyDiagnostic/ReportNonSafetyDiagnostic instead", error: false)]
        private void ReportDiagnostic(ErrorCode errorCode, SyntaxNode syntaxNode, params object[] arguments)
        {
            Debug.Assert(!IsConditionalState);
            if (this.State.Reachable && !_disableDiagnostics)
            {
                Diagnostics.Add(errorCode, syntaxNode.GetLocation(), arguments);
            }
        }

        private void InheritNullableStateOfTrackableStruct(TypeSymbol targetType, int targetSlot, int valueSlot, bool isDefaultValue, int skipSlot = -1)
        {
            Debug.Assert(targetSlot > 0);
            Debug.Assert(EmptyStructTypeCache.IsTrackableStructType(targetType));

            if (skipSlot < 0)
            {
                skipSlot = targetSlot;
            }

            // https://github.com/dotnet/roslyn/issues/29619 Handle properties not backed by fields.
            // See ModifyMembers_StructPropertyNoBackingField and PropertyCycle_Struct tests.
            foreach (var field in _emptyStructTypeCache.GetStructInstanceFields(targetType))
            {
                InheritNullableStateOfMember(targetSlot, valueSlot, field, isDefaultValue: isDefaultValue, skipSlot);
            }
        }

        // 'skipSlot' is the original target slot that should be skipped in case of cycles.
        private void InheritNullableStateOfMember(int targetContainerSlot, int valueContainerSlot, Symbol member, bool isDefaultValue, int skipSlot)
        {
            Debug.Assert(targetContainerSlot > 0);
            Debug.Assert(skipSlot > 0);
            // https://github.com/dotnet/roslyn/issues/33428: Ensure member is valid for target and value.

            TypeWithAnnotations fieldOrPropertyType = member.GetTypeOrReturnType();

            // Nullable<T> is handled here rather than in InheritNullableStateOfTrackableStruct since that
            // method only clones auto-properties (see https://github.com/dotnet/roslyn/issues/29619).
            // When that issue is fixed, Nullable<T> should be handled there instead.
            if (fieldOrPropertyType.Type.IsReferenceType ||
                fieldOrPropertyType.TypeKind == TypeKind.TypeParameter ||
                fieldOrPropertyType.IsNullableType())
            {
                int targetMemberSlot = GetOrCreateSlot(member, targetContainerSlot);
                Debug.Assert(targetMemberSlot > 0);

                NullableFlowState value = isDefaultValue ? NullableFlowState.MaybeNull : fieldOrPropertyType.ToTypeWithState().State;
                int valueMemberSlot = -1;

                if (valueContainerSlot > 0)
                {
                    valueMemberSlot = VariableSlot(member, valueContainerSlot);
                    if (valueMemberSlot == skipSlot)
                    {
                        return;
                    }
                    value = valueMemberSlot > 0 && valueMemberSlot < this.State.Capacity ?
                        this.State[valueMemberSlot] :
                        NullableFlowState.NotNull;
                }

                SetStateAndTrackForFinally(ref this.State, targetMemberSlot, value);
                if (valueMemberSlot > 0)
                {
                    InheritNullableStateOfTrackableType(targetMemberSlot, valueMemberSlot, skipSlot);
                }
            }
            else if (EmptyStructTypeCache.IsTrackableStructType(fieldOrPropertyType.Type))
            {
                int targetMemberSlot = GetOrCreateSlot(member, targetContainerSlot);
                if (targetMemberSlot > 0)
                {
                    int valueMemberSlot = (valueContainerSlot > 0) ? GetOrCreateSlot(member, valueContainerSlot) : -1;
                    if (valueMemberSlot == skipSlot)
                    {
                        return;
                    }
                    InheritNullableStateOfTrackableStruct(fieldOrPropertyType.Type, targetMemberSlot, valueMemberSlot, isDefaultValue: isDefaultValue, skipSlot);
                }
            }
        }

        /// <summary>
        /// Whenever setting the state of a variable, and that variable is not declared at the point the state is being set,
        /// and the new state might be <see cref="NullableFlowState.MaybeNull"/>, this method should be called to perform the
        /// state setting and to ensure the mutation is visible outside the finally block when the mutation occurs in a finally block.
        /// </summary>
        private void SetStateAndTrackForFinally(ref LocalState state, int slot, NullableFlowState newState)
        {
            state[slot] = newState;
            if (newState == NullableFlowState.MaybeNull && _tryState.HasValue)
            {
                var tryState = _tryState.Value;
                tryState[slot] = NullableFlowState.MaybeNull;
                _tryState = tryState;
            }
        }

        private void InheritDefaultState(int targetSlot)
        {
            Debug.Assert(targetSlot > 0);

            // Reset the state of any members of the target.
            for (int slot = targetSlot + 1; slot < nextVariableSlot; slot++)
            {
                var variable = variableBySlot[slot];
                if (variable.ContainingSlot != targetSlot)
                {
                    continue;
                }
                SetStateAndTrackForFinally(ref this.State, slot, variable.Symbol.GetTypeOrReturnType().ToTypeWithState().State);
                InheritDefaultState(slot);
            }
        }

        private void InheritNullableStateOfTrackableType(int targetSlot, int valueSlot, int skipSlot)
        {
            Debug.Assert(targetSlot > 0);
            Debug.Assert(valueSlot > 0);

            // Clone the state for members that have been set on the value.
            for (int slot = valueSlot + 1; slot < nextVariableSlot; slot++)
            {
                var variable = variableBySlot[slot];
                if (variable.ContainingSlot != valueSlot)
                {
                    continue;
                }
                var member = variable.Symbol;
                Debug.Assert(member.Kind == SymbolKind.Field || member.Kind == SymbolKind.Property || member.Kind == SymbolKind.Event);
                InheritNullableStateOfMember(targetSlot, valueSlot, member, isDefaultValue: false, skipSlot);
            }
        }

        private TypeSymbol GetSlotType(int slot)
        {
            return VariableTypeWithAnnotations(variableBySlot[slot].Symbol).Type;
        }

        protected override LocalState TopState()
        {
            var state = new LocalState(reachable: true, new ArrayBuilder<NullableFlowState>(nextVariableSlot));
            Populate(ref state, start: 0);
            return state;
        }

        protected override LocalState UnreachableState()
        {
            return new LocalState(reachable: false, null);
        }

        protected override LocalState ReachableBottomState()
        {
            // Create a reachable state in which all variables are known to be non-null.
            var builder = new ArrayBuilder<NullableFlowState>(nextVariableSlot);
            builder.AddMany(NullableFlowState.NotNull, nextVariableSlot);
            return new LocalState(reachable: true, builder);
        }

        private void EnterParameters()
        {
            var methodSymbol = _symbol as MethodSymbol;
            if (methodSymbol is null)
            {
                return;
            }

            var methodParameters = methodSymbol.Parameters;
            var signatureParameters = _useMethodSignatureParameterTypes ? _methodSignatureOpt.Parameters : methodParameters;
            Debug.Assert(signatureParameters.Length == methodParameters.Length);
            int n = methodParameters.Length;
            for (int i = 0; i < n; i++)
            {
                var parameter = methodParameters[i];
                var parameterType = signatureParameters[i].TypeWithAnnotations;
                EnterParameter(parameter, parameterType);
            }
        }

        private void EnterParameter(ParameterSymbol parameter, TypeWithAnnotations parameterType)
        {
            _variableTypes[parameter] = parameterType;
            int slot = GetOrCreateSlot(parameter);

            Debug.Assert(!IsConditionalState);
            if (slot > 0 && parameter.RefKind != RefKind.Out)
            {
                if (EmptyStructTypeCache.IsTrackableStructType(parameterType.Type))
                {
                    InheritNullableStateOfTrackableStruct(
                        parameterType.Type,
                        slot,
                        valueSlot: -1,
                        isDefaultValue: parameter.ExplicitDefaultConstantValue?.IsNull == true);
                }
            }
        }

        public override BoundNode VisitIsPatternExpression(BoundIsPatternExpression node)
        {
            var resultType = VisitRvalueWithState(node.Expression);
            VisitPattern(node.Expression, resultType, node.Pattern);
            SetNotNullResult(node);
            return node;
        }

        /// <summary>
        /// Examples:
        /// `x is Point p`
        /// `switch (x) ... case Point p:` // https://github.com/dotnet/roslyn/issues/29873 not yet handled
        ///
        /// If the expression is trackable, we'll return with different null-states for that expression in the two conditional states.
        /// If the pattern is a `var` pattern, we'll also have re-inferred the `var` type with nullability and
        /// updated the state for that declared local.
        /// </summary>
        private void VisitPattern(BoundExpression expression, TypeWithState expressionResultType, BoundPattern pattern)
        {
            NullableFlowState whenTrue = expressionResultType.State;
            NullableFlowState whenFalse = expressionResultType.State;

            switch (pattern.Kind)
            {
                case BoundKind.ConstantPattern:
                    // If the constant is null, the pattern tells us the expression is null.
                    // If the constant is not null, the pattern tells us the expression is not null.
                    // If there is no constant, we don't know.
                    switch (((BoundConstantPattern)pattern).ConstantValue?.IsNull)
                    {
                        case true:
                            whenTrue = NullableFlowState.MaybeNull;
                            whenFalse = NullableFlowState.NotNull;
                            break;
                        case false:
                            whenTrue = NullableFlowState.NotNull;
                            whenFalse = NullableFlowState.MaybeNull;
                            break;
                    }
                    break;
                case BoundKind.DeclarationPattern:
                    var declarationPattern = (BoundDeclarationPattern)pattern;
                    if (declarationPattern.IsVar)
                    {
                        // The result type and state of the expression carry into the variable declared by var pattern
                        Symbol variable = declarationPattern.Variable;
                        // No variable declared for discard (`i is var _`)
                        if ((object)variable != null)
                        {
                            var variableType = expressionResultType.ToTypeWithAnnotations();
                            _variableTypes[variable] = variableType;
                            TrackNullableStateForAssignment(expression, variableType, GetOrCreateSlot(variable), expressionResultType);
                        }

                        whenFalse = NullableFlowState.NotNull; // whenFalse is unreachable
                    }
                    else
                    {
                        whenTrue = NullableFlowState.NotNull; // the pattern tells us the expression is not null
                    }
                    break;
                default:
                    // https://github.com/dotnet/roslyn/issues/29909 : handle other kinds of patterns
                    break;
            }

            Debug.Assert(!IsConditionalState);

            // Create slot since EnsureCapacity should be
            // called on all fields and that is simpler if state is limited to this.State.
            int mainSlot = MakeSlot(expression);

            base.VisitPattern(pattern);
            Debug.Assert(IsConditionalState);

            if (mainSlot > 0)
            {
                SetStateAndTrackForFinally(ref this.StateWhenTrue, mainSlot, whenTrue);
                SetStateAndTrackForFinally(ref this.StateWhenFalse, mainSlot, whenFalse);
            }

            if (whenTrue.IsNotNull() || whenFalse.IsNotNull())
            {
                var slotBuilder = ArrayBuilder<int>.GetInstance();
                GetSlotsToMarkAsNotNullable(expression, slotBuilder);

                // Set all nested conditional slots. For example in a?.b?.c we'll set a, b, and c.
                if (whenTrue.IsNotNull())
                {
                    MarkSlotsAsNotNull(slotBuilder, ref StateWhenTrue);
                }
                else if (whenFalse.IsNotNull())
                {
                    MarkSlotsAsNotNull(slotBuilder, ref StateWhenFalse);
                }

                slotBuilder.Free();
            }
        }

        protected override BoundNode VisitReturnStatementNoAdjust(BoundReturnStatement node)
        {
            Debug.Assert(!IsConditionalState);

            BoundExpression expr = node.ExpressionOpt;
            if (expr == null)
            {
                return null;
            }

            // Should not convert to method return type when inferring return type (when _returnTypesOpt != null).
            if (_returnTypesOpt == null &&
                TryGetReturnType(out TypeWithAnnotations returnType))
            {
                if (node.RefKind == RefKind.None)
                {
                    VisitOptionalImplicitConversion(expr, returnType, useLegacyWarnings: false, AssignmentKind.Return);
                }
                else
                {
                    // return ref expr;
                    VisitRefExpression(expr, returnType);
                }
            }
            else
            {
                var result = VisitRvalueWithState(expr);
                if (_returnTypesOpt != null)
                {
                    _returnTypesOpt.Add((node, result.ToTypeWithAnnotations()));
                }
            }

            return null;
        }

        private TypeWithState VisitRefExpression(BoundExpression expr, TypeWithAnnotations destinationType)
        {
            Visit(expr);
            TypeWithState resultType = ResultType;
            if (!expr.IsSuppressed && RemoveConversion(expr, includeExplicitConversions: false).expression.Kind != BoundKind.ThrowExpression)
            {
                var lvalueResultType = LvalueResultType;
                if (IsNullabilityMismatch(lvalueResultType, destinationType))
                {
                    // declared types must match
                    ReportNullabilityMismatchInAssignment(expr.Syntax, lvalueResultType, destinationType);
                }
                else
                {
                    // types match, but state would let a null in
                    ReportNullableAssignmentIfNecessary(expr, destinationType, resultType, useLegacyWarnings: false);
                }
            }

            return resultType;
        }

        private bool TryGetReturnType(out TypeWithAnnotations type)
        {
            var method = (MethodSymbol)_symbol;
            var returnType = (_useMethodSignatureReturnType ? _methodSignatureOpt : method).ReturnTypeWithAnnotations;
            Debug.Assert((object)returnType != LambdaSymbol.ReturnTypeIsBeingInferred);

            if (returnType.SpecialType == SpecialType.System_Void)
            {
                type = default;
                return false;
            }

            if (!method.IsAsync)
            {
                type = returnType;
                return true;
            }

            if (method.IsGenericTaskReturningAsync(compilation))
            {
                type = ((NamedTypeSymbol)returnType.Type).TypeArgumentsWithAnnotationsNoUseSiteDiagnostics.Single();
                return true;
            }

            type = default;
            return false;
        }

        private static bool RequiresSafetyWarningWhenNullIntroduced(TypeSymbol typeOpt)
        {
            return typeOpt?.IsTypeParameterDisallowingAnnotation() == true && !typeOpt.IsNullableTypeOrTypeParameter();
        }

        public override BoundNode VisitLocal(BoundLocal node)
        {
            var local = node.LocalSymbol;
            int slot = GetOrCreateSlot(local);
            var type = GetDeclaredLocalResult(local);
            SetResult(GetAdjustedResult(type, slot), type);
            return null;
        }

        public override BoundNode VisitLocalDeclaration(BoundLocalDeclaration node)
        {
            var local = node.LocalSymbol;
            int slot = GetOrCreateSlot(local);

            var initializer = node.InitializerOpt;
            if (initializer is null)
            {
                return null;
            }

            TypeWithAnnotations type = local.TypeWithAnnotations;
            TypeWithState valueType;
            if (local.IsRef)
            {
                valueType = VisitRefExpression(initializer, type);
            }
            else
            {
                bool inferredType = node.DeclaredType.InferredType;
                valueType = VisitOptionalImplicitConversion(initializer, targetTypeOpt: inferredType ? default : type, useLegacyWarnings: true, AssignmentKind.Assignment);
                if (inferredType)
                {
                    if (valueType.HasNullType)
                    {
                        Debug.Assert(type.Type.IsErrorType());
                        valueType = type.ToTypeWithState();
                    }

                    type = valueType.ToTypeWithAnnotations();
                    _variableTypes[local] = type;
                }
            }

            TrackNullableStateForAssignment(initializer, type, slot, valueType, MakeSlot(initializer));
            return null;
        }

        protected override BoundExpression VisitExpressionWithoutStackGuard(BoundExpression node)
        {
            Debug.Assert(!IsConditionalState);
            bool wasReachable = this.State.Reachable;
            ResultType = _invalidType;
            _ = base.VisitExpressionWithoutStackGuard(node);
            TypeWithState resultType = ResultType;

#if DEBUG
            // Verify Visit method set _result.
            Debug.Assert((object)resultType.Type != _invalidType.Type);
            Debug.Assert(AreCloseEnough(resultType.Type, node.Type));
#endif
            if (node.IsSuppressed || node.HasAnyErrors || !wasReachable)
            {
                resultType = resultType.WithNotNullState();
                SetResult(resultType, LvalueResultType);
            }

            _callbackOpt?.Invoke(node, resultType.ToTypeWithAnnotations());
            return null;
        }

#if DEBUG
        // For asserts only.
        private static bool AreCloseEnough(TypeSymbol typeA, TypeSymbol typeB)
        {
            if ((object)typeA == typeB)
            {
                return true;
            }
            if (typeA is null || typeB is null)
            {
                return typeA?.IsErrorType() != false && typeB?.IsErrorType() != false;
            }
            return canIgnoreAnyType(typeA) ||
                canIgnoreAnyType(typeB) ||
                typeA.Equals(typeB, TypeCompareKind.IgnoreCustomModifiersAndArraySizesAndLowerBounds | TypeCompareKind.IgnoreNullableModifiersForReferenceTypes | TypeCompareKind.IgnoreDynamicAndTupleNames); // Ignore TupleElementNames (see https://github.com/dotnet/roslyn/issues/23651).

            bool canIgnoreAnyType(TypeSymbol type)
            {
                return (object)type.VisitType((t, unused1, unused2) => canIgnoreType(t), (object)null) != null;
            }
            bool canIgnoreType(TypeSymbol type)
            {
                return type.IsErrorType() || type.IsDynamic() || type.HasUseSiteError || (type.IsAnonymousType && canIgnoreAnonymousType((NamedTypeSymbol)type));
            }
            bool canIgnoreAnonymousType(NamedTypeSymbol type)
            {
                return AnonymousTypeManager.GetAnonymousTypePropertyTypesWithAnnotations(type).Any(t => canIgnoreAnyType(t.Type));
            }
        }
#endif

        protected override void VisitStatement(BoundStatement statement)
        {
            ResultType = _invalidType;
            base.VisitStatement(statement);
            ResultType = _invalidType;
        }

        public override BoundNode VisitObjectCreationExpression(BoundObjectCreationExpression node)
        {
            Debug.Assert(!IsConditionalState);
            var arguments = node.Arguments;
            var argumentResults = VisitArguments(node, arguments, node.ArgumentRefKindsOpt, node.Constructor, node.ArgsToParamsOpt, node.Expanded);
            VisitObjectOrDynamicObjectCreation(node, arguments, argumentResults, node.InitializerExpressionOpt);
            return null;
        }

        private void VisitObjectOrDynamicObjectCreation(
            BoundExpression node,
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<VisitResult> argumentResults,
            BoundExpression initializerOpt)
        {
            Debug.Assert(node.Kind == BoundKind.ObjectCreationExpression ||
                node.Kind == BoundKind.DynamicObjectCreationExpression ||
                node.Kind == BoundKind.NewT);
            var argumentTypes = argumentResults.SelectAsArray(ar => ar.RValueType);

            int slot = -1;
            TypeSymbol type = node.Type;
            NullableFlowState resultState = NullableFlowState.NotNull;
            if ((object)type != null)
            {
                slot = GetOrCreateObjectCreationPlaceholderSlot(node);
                if (slot > 0)
                {
                    var constructor = (node as BoundObjectCreationExpression)?.Constructor;
                    bool isDefaultValueTypeConstructor = constructor?.IsDefaultValueTypeConstructor() == true;

                    if (EmptyStructTypeCache.IsTrackableStructType(type))
                    {
                        this.State[slot] = NullableFlowState.NotNull;
                        var tupleType = constructor?.ContainingType as TupleTypeSymbol;
                        if ((object)tupleType != null && !isDefaultValueTypeConstructor)
                        {
                            // new System.ValueTuple<T1, ..., TN>(e1, ..., eN)
                            TrackNullableStateOfTupleElements(slot, tupleType, arguments, argumentTypes, useRestField: true);
                        }
                        else
                        {
                            InheritNullableStateOfTrackableStruct(
                                type,
                                slot,
                                valueSlot: -1,
                                isDefaultValue: isDefaultValueTypeConstructor);
                        }
                    }
                    else if (type.IsNullableType())
                    {
                        if (isDefaultValueTypeConstructor)
                        {
                            // a nullable value type created with its default constructor is by definition null
                            resultState = NullableFlowState.MaybeNull;
                        }
                        else if (constructor.ParameterCount == 1)
                        {
                            // if we deal with one-parameter ctor that takes underlying, then Value state is inferred from the argument.
                            var parameterType = constructor.ParameterTypesWithAnnotations[0];
                            if (AreNullableAndUnderlyingTypes(type, parameterType.Type, out TypeWithAnnotations underlyingType))
                            {
                                TrackNullableStateOfNullableValue(node, arguments[0], type, underlyingType);
                            }
                        }
                    }
                }
            }

            if (initializerOpt != null)
            {
                VisitObjectCreationInitializer(null, slot, initializerOpt);
            }

            ResultType = new TypeWithState(type, resultState);
        }

        private void VisitObjectCreationInitializer(Symbol containingSymbol, int containingSlot, BoundExpression node)
        {
            switch (node.Kind)
            {
                case BoundKind.ObjectInitializerExpression:
                    foreach (var initializer in ((BoundObjectInitializerExpression)node).Initializers)
                    {
                        switch (initializer.Kind)
                        {
                            case BoundKind.AssignmentOperator:
                                VisitObjectElementInitializer(containingSlot, (BoundAssignmentOperator)initializer);
                                break;
                            default:
                                VisitRvalue(initializer);
                                break;
                        }
                    }
                    break;
                case BoundKind.CollectionInitializerExpression:
                    foreach (var initializer in ((BoundCollectionInitializerExpression)node).Initializers)
                    {
                        switch (initializer.Kind)
                        {
                            case BoundKind.CollectionElementInitializer:
                                VisitCollectionElementInitializer((BoundCollectionElementInitializer)initializer);
                                break;
                            default:
                                VisitRvalue(initializer);
                                break;
                        }
                    }
                    break;
                default:
                    TypeWithState resultType = VisitRvalueWithState(node);
                    Debug.Assert((object)containingSymbol != null);
                    if ((object)containingSymbol != null)
                    {
                        var type = containingSymbol.GetTypeOrReturnType();
                        ReportAssignmentWarnings(node, type, resultType, useLegacyWarnings: false);
                        TrackNullableStateForAssignment(node, type, containingSlot, resultType, MakeSlot(node));
                    }
                    break;
            }
        }

        private void VisitObjectElementInitializer(int containingSlot, BoundAssignmentOperator node)
        {
            var left = node.Left;
            switch (left.Kind)
            {
                case BoundKind.ObjectInitializerMember:
                    {
                        var objectInitializer = (BoundObjectInitializerMember)left;
                        var symbol = objectInitializer.MemberSymbol;
                        if (!objectInitializer.Arguments.IsDefaultOrEmpty)
                        {
                            VisitArguments(objectInitializer, objectInitializer.Arguments, objectInitializer.ArgumentRefKindsOpt, (PropertySymbol)symbol, objectInitializer.ArgsToParamsOpt, objectInitializer.Expanded);
                        }
                        if ((object)symbol != null)
                        {
                            int slot = (containingSlot < 0) ? -1 : GetOrCreateSlot(symbol, containingSlot);
                            VisitObjectCreationInitializer(symbol, slot, node.Right);
                        }
                    }
                    break;
                default:
                    Visit(node);
                    break;
            }
        }

        private new void VisitCollectionElementInitializer(BoundCollectionElementInitializer node)
        {
            // Note: we analyze even omitted calls
            VisitArguments(node, node.Arguments, refKindsOpt: default, node.AddMethod, node.ArgsToParamsOpt, node.Expanded);
            SetUnknownResultNullability();
        }

        private void SetNotNullResult(BoundExpression node)
        {
            ResultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
        }

        private int GetOrCreateObjectCreationPlaceholderSlot(BoundExpression node)
        {
            ObjectCreationPlaceholderLocal placeholder;
            if (_placeholderLocalsOpt == null)
            {
                _placeholderLocalsOpt = PooledDictionary<BoundExpression, ObjectCreationPlaceholderLocal>.GetInstance();
                placeholder = null;
            }
            else
            {
                _placeholderLocalsOpt.TryGetValue(node, out placeholder);
            }

            if (placeholder is null)
            {
                placeholder = new ObjectCreationPlaceholderLocal(_symbol, node);
                _placeholderLocalsOpt.Add(node, placeholder);
            }

            return GetOrCreateSlot(placeholder);
        }

        public override BoundNode VisitAnonymousObjectCreationExpression(BoundAnonymousObjectCreationExpression node)
        {
            Debug.Assert(!IsConditionalState);

            var anonymousType = (NamedTypeSymbol)node.Type;
            Debug.Assert(anonymousType.IsAnonymousType);

            var arguments = node.Arguments;
            var argumentTypes = arguments.SelectAsArray((arg, self) =>
                self.VisitRvalueWithState(arg), this);
            var argumentsWithAnnotations = argumentTypes.SelectAsArray((arg, self) =>
                arg.ToTypeWithAnnotations(), this);

            if (argumentsWithAnnotations.All(argType => argType.HasType))
            {
                anonymousType = AnonymousTypeManager.ConstructAnonymousTypeSymbol(anonymousType, argumentsWithAnnotations);
                int receiverSlot = GetOrCreateObjectCreationPlaceholderSlot(node);
                for (int i = 0; i < arguments.Length; i++)
                {
                    var argument = arguments[i];
                    var argumentType = argumentTypes[i];
                    var property = AnonymousTypeManager.GetAnonymousTypeProperty(anonymousType, i);
                    TrackNullableStateForAssignment(argument, property.TypeWithAnnotations, GetOrCreateSlot(property, receiverSlot), argumentType, MakeSlot(argument));
                }
            }

            ResultType = new TypeWithState(anonymousType, NullableFlowState.NotNull);
            return null;
        }

        public override BoundNode VisitArrayCreation(BoundArrayCreation node)
        {
            foreach (var expr in node.Bounds)
            {
                VisitRvalue(expr);
            }
            TypeSymbol resultType = (node.InitializerOpt == null) ? node.Type : VisitArrayInitializer(node);
            ResultType = new TypeWithState(resultType, NullableFlowState.NotNull);
            return null;
        }

        private ArrayTypeSymbol VisitArrayInitializer(BoundArrayCreation node)
        {
            BoundArrayInitialization initialization = node.InitializerOpt;
            var expressions = ArrayBuilder<BoundExpression>.GetInstance(initialization.Initializers.Length);
            GetArrayElements(initialization, expressions);
            int n = expressions.Count;

            // Consider recording in the BoundArrayCreation
            // whether the array was implicitly typed, rather than relying on syntax.
            bool isInferred = node.Syntax.Kind() == SyntaxKind.ImplicitArrayCreationExpression;
            var arrayType = (ArrayTypeSymbol)node.Type;
            var elementType = arrayType.ElementTypeWithAnnotations;
            if (!isInferred)
            {
                for (int i = 0; i < n; i++)
                {
                    _ = VisitOptionalImplicitConversion(expressions[i], elementType, useLegacyWarnings: false, AssignmentKind.Assignment);
                }

                ResultType = _invalidType;
                return arrayType;
            }

            var conversions = ArrayBuilder<Conversion>.GetInstance(n);
            var resultTypes = ArrayBuilder<TypeWithState>.GetInstance(n);
            for (int i = 0; i < n; i++)
            {
                // collect expressions, conversions and result types
                (BoundExpression expression, Conversion conversion) = RemoveConversion(expressions[i], includeExplicitConversions: false);
                expressions[i] = expression;
                conversions.Add(conversion);
                var resultType = VisitRvalueWithState(expression);
                resultTypes.Add(resultType);
            }

            var placeholderBuilder = ArrayBuilder<BoundExpression>.GetInstance(n);
            for (int i = 0; i < n; i++)
            {
                placeholderBuilder.Add(CreatePlaceholderIfNecessary(expressions[i], resultTypes[i].ToTypeWithAnnotations()));
            }
            var placeholders = placeholderBuilder.ToImmutableAndFree();

            TypeSymbol bestType = null;
            if (!node.HasErrors)
            {
                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                bestType = BestTypeInferrer.InferBestType(placeholders, _conversions, ref useSiteDiagnostics);
            }

            TypeWithAnnotations inferredType;
            if (bestType is null)
            {
                inferredType = elementType.SetUnknownNullabilityForReferenceTypes();
            }
            else
            {
                inferredType = TypeWithAnnotations.Create(bestType);
            }

            if ((object)bestType != null)
            {
                // Convert elements to best type to determine element top-level nullability and to report nested nullability warnings
                for (int i = 0; i < n; i++)
                {
                    var placeholder = placeholders[i];
                    resultTypes[i] = ApplyConversion(placeholder, placeholder, conversions[i], inferredType, resultTypes[i], checkConversion: true,
                        fromExplicitCast: false, useLegacyWarnings: false, AssignmentKind.Assignment, reportRemainingWarnings: true, reportTopLevelWarnings: false);
                }

                // Set top-level nullability on inferred element type
                inferredType = TypeWithAnnotations.Create(inferredType.Type, BestTypeInferrer.GetNullableAnnotation(resultTypes));

                for (int i = 0; i < n; i++)
                {
                    var nodeForSyntax = expressions[i];
                    // Report top-level warnings
                    _ = ApplyConversion(nodeForSyntax, operandOpt: nodeForSyntax, Conversion.Identity, targetTypeWithNullability: inferredType, operandType: resultTypes[i],
                        checkConversion: true, fromExplicitCast: false, useLegacyWarnings: false, AssignmentKind.Assignment, reportRemainingWarnings: false);
                }
            }

            resultTypes.Free();
            expressions.Free();

            ResultType = _invalidType;
            arrayType = arrayType.WithElementType(inferredType);
            return arrayType;
        }

        /// <summary>
        /// Applies a method similar to <see cref="VisitArrayInitializer(BoundArrayCreation)"/>
        /// The expressions returned from a lambda are not converted though, so we'll have to classify fresh conversions.
        /// Note: even if some conversions fail, we'll proceed to infer top-level nullability. That is reasonable in common cases.
        /// </summary>
        internal static TypeWithAnnotations BestTypeForLambdaReturns(
            ArrayBuilder<(BoundExpression, TypeWithAnnotations)> returns,
            CSharpCompilation compilation,
            BoundNode node)
        {
            var walker = new NullableWalker(compilation, method: null,
                useMethodSignatureReturnType: false, useMethodSignatureParameterTypes: false, methodSignatureOpt: null,
                node, returnTypesOpt: null, initialState: null, callbackOpt: null);

            int n = returns.Count;
            var resultTypes = ArrayBuilder<TypeWithAnnotations>.GetInstance(n);
            var placeholdersBuilder = ArrayBuilder<BoundExpression>.GetInstance(n);
            for (int i = 0; i < n; i++)
            {
                var (returnExpr, resultType) = returns[i];
                resultTypes.Add(resultType);
                placeholdersBuilder.Add(CreatePlaceholderIfNecessary(returnExpr, resultType));
            }

            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            var placeholders = placeholdersBuilder.ToImmutableAndFree();
            TypeSymbol bestType = BestTypeInferrer.InferBestType(placeholders, walker._conversions, ref useSiteDiagnostics);

            TypeWithAnnotations inferredType;
            if ((object)bestType != null)
            {
                // Note: so long as we have a best type, we can proceed.
                var bestTypeWithObliviousAnnotation = TypeWithAnnotations.Create(bestType);
                ConversionsBase conversionsWithoutNullability = walker._conversions.WithNullability(false);
                for (int i = 0; i < n; i++)
                {
                    BoundExpression placeholder = placeholders[i];
                    Conversion conversion = conversionsWithoutNullability.ClassifyConversionFromExpression(placeholder, bestType, ref useSiteDiagnostics);
                    resultTypes[i] = walker.ApplyConversion(placeholder, placeholder, conversion, bestTypeWithObliviousAnnotation, resultTypes[i].ToTypeWithState(),
                        checkConversion: false, fromExplicitCast: false, useLegacyWarnings: false, AssignmentKind.Return,
                        reportRemainingWarnings: false, reportTopLevelWarnings: false).ToTypeWithAnnotations();
                }

                // Set top-level nullability on inferred type
                inferredType = TypeWithAnnotations.Create(bestType, BestTypeInferrer.GetNullableAnnotation(resultTypes));
            }
            else
            {
                inferredType = default;
            }

            resultTypes.Free();
            walker.Free();

            return inferredType;
        }

        private static void GetArrayElements(BoundArrayInitialization node, ArrayBuilder<BoundExpression> builder)
        {
            foreach (var child in node.Initializers)
            {
                if (child.Kind == BoundKind.ArrayInitialization)
                {
                    GetArrayElements((BoundArrayInitialization)child, builder);
                }
                else
                {
                    builder.Add(child);
                }
            }
        }

        public override BoundNode VisitArrayAccess(BoundArrayAccess node)
        {
            Debug.Assert(!IsConditionalState);

            Visit(node.Expression);

            Debug.Assert(!IsConditionalState);
            Debug.Assert(!node.Expression.Type.IsValueType);
            // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
            // after indices have been visited, and only if the receiver has not changed.
            CheckPossibleNullReceiver(node.Expression);

            var type = ResultType.Type as ArrayTypeSymbol;

            foreach (var i in node.Indices)
            {
                VisitRvalue(i);
            }

            TypeWithAnnotations result;
            if (node.Indices.Length == 1 &&
                TypeSymbol.Equals(node.Indices[0].Type, compilation.GetWellKnownType(WellKnownType.System_Range), TypeCompareKind.ConsiderEverything2))
            {
                result = TypeWithAnnotations.Create(type);
            }
            else
            {
                result = type?.ElementTypeWithAnnotations ?? default;
            }
            LvalueResultType = result;

            return null;
        }

        private TypeWithState InferResultNullability(BoundBinaryOperator node, TypeWithState leftType, TypeWithState rightType)
        {
            return InferResultNullability(node.OperatorKind, node.MethodOpt, node.Type, leftType, rightType);
        }

        private TypeWithState InferResultNullability(BinaryOperatorKind operatorKind, MethodSymbol methodOpt, TypeSymbol resultType, TypeWithState leftType, TypeWithState rightType)
        {
            NullableFlowState resultState = NullableFlowState.NotNull;
            if (operatorKind.IsUserDefined())
            {
                // Update method based on operand types: see https://github.com/dotnet/roslyn/issues/29605.
                if ((object)methodOpt != null && methodOpt.ParameterCount == 2)
                {
                    return operatorKind.IsLifted() && !operatorKind.IsComparison()
                        ? LiftedReturnType(methodOpt.ReturnTypeWithAnnotations, leftType.State.Join(rightType.State))
                        : methodOpt.ReturnTypeWithAnnotations.ToTypeWithState();
                }
            }
            else if (!operatorKind.IsDynamic() && !resultType.IsValueType)
            {
                switch (operatorKind.Operator() | operatorKind.OperandTypes())
                {
                    case BinaryOperatorKind.DelegateCombination:
                        resultState = leftType.State.Meet(rightType.State);
                        break;
                    case BinaryOperatorKind.DelegateRemoval:
                        resultState = NullableFlowState.MaybeNull; // Delegate removal can produce null.
                        break;
                    default:
                        resultState = NullableFlowState.NotNull;
                        break;
                }
            }

            if (operatorKind.IsLifted())
            {
                resultState = leftType.State.Join(rightType.State);
            }

            return new TypeWithState(resultType, resultState);
        }

        protected override void AfterLeftChildHasBeenVisited(BoundBinaryOperator binary)
        {
            Debug.Assert(!IsConditionalState);
            TypeWithState leftType = ResultType;

            var rightType = VisitRvalueWithState(binary.Right);
            Debug.Assert(!IsConditionalState);
            // At this point, State.Reachable may be false for
            // invalid code such as `s + throw new Exception()`.

            if (binary.OperatorKind.IsUserDefined() && binary.MethodOpt?.ParameterCount == 2)
            {
                var parameters = binary.MethodOpt.Parameters;
                ReportArgumentWarnings(binary.Left, leftType, parameters[0]);
                ReportArgumentWarnings(binary.Right, rightType, parameters[1]);
            }

            Debug.Assert(!IsConditionalState);
            ResultType = InferResultNullability(binary, leftType, rightType);

            BinaryOperatorKind op = binary.OperatorKind.Operator();

            // learn from non-null constant
            BoundExpression operandComparedToNonNull = null;
            if (isNonNullConstant(binary.Left))
            {
                operandComparedToNonNull = binary.Right;
            }
            else if (isNonNullConstant(binary.Right))
            {
                operandComparedToNonNull = binary.Left;
            }

            if (operandComparedToNonNull != null)
            {
                switch (op)
                {
                    case BinaryOperatorKind.Equal:
                    case BinaryOperatorKind.GreaterThan:
                    case BinaryOperatorKind.LessThan:
                    case BinaryOperatorKind.GreaterThanOrEqual:
                    case BinaryOperatorKind.LessThanOrEqual:
                        operandComparedToNonNull = SkipReferenceConversions(operandComparedToNonNull);
                        splitAndLearnFromNonNullTest(operandComparedToNonNull, whenTrue: true);
                        return;
                    case BinaryOperatorKind.NotEqual:
                        operandComparedToNonNull = SkipReferenceConversions(operandComparedToNonNull);
                        splitAndLearnFromNonNullTest(operandComparedToNonNull, whenTrue: false);
                        return;
                    default:
                        break;
                };
            }

            // learn from null constant
            if (op == BinaryOperatorKind.Equal || op == BinaryOperatorKind.NotEqual)
            {
                BoundExpression operandComparedToNull = null;

                if (binary.Right.ConstantValue?.IsNull == true)
                {
                    operandComparedToNull = binary.Left;
                }
                else if (binary.Left.ConstantValue?.IsNull == true)
                {
                    operandComparedToNull = binary.Right;
                }

                if (operandComparedToNull != null)
                {
                    operandComparedToNull = SkipReferenceConversions(operandComparedToNull);

                    // Set all nested conditional slots. For example in a?.b?.c we'll set a, b, and c.
                    bool nonNullCase = op != BinaryOperatorKind.Equal; // true represents WhenTrue
                    splitAndLearnFromNonNullTest(operandComparedToNull, whenTrue: nonNullCase);

                    // `x == null` and `x != null` are pure null tests so update the null-state in the alternative branch too
                    LearnFromNullTest(operandComparedToNull, ref nonNullCase ? ref StateWhenFalse : ref StateWhenTrue);
                }
            }

            static BoundExpression skipImplicitNullableConversions(BoundExpression possiblyConversion)
            {
                while (possiblyConversion.Kind == BoundKind.Conversion &&
                    possiblyConversion is BoundConversion { ConversionKind: ConversionKind.ImplicitNullable, Operand: var operand })
                {
                    possiblyConversion = operand;
                }
                return possiblyConversion;
            }

            void splitAndLearnFromNonNullTest(BoundExpression operandComparedToNull, bool whenTrue)
            {
                var slotBuilder = ArrayBuilder<int>.GetInstance();
                GetSlotsToMarkAsNotNullable(operandComparedToNull, slotBuilder);
                if (slotBuilder.Count != 0)
                {
                    Split();
                    ref LocalState stateToUpdate = ref whenTrue ? ref this.StateWhenTrue : ref this.StateWhenFalse;
                    MarkSlotsAsNotNull(slotBuilder, ref stateToUpdate);
                }
                slotBuilder.Free();
            }

            static bool isNonNullConstant(BoundExpression expr)
                => skipImplicitNullableConversions(expr).ConstantValue?.IsNull == false;
        }

        /// <summary>
        /// If we learn that the operand is non-null, we can infer that certain
        /// sub-expressions were also non-null.
        /// Get all nested conditional slots for those sub-expressions. For example in a?.b?.c we'll set a, b, and c.
        /// Only returns slots for tracked expressions.
        /// </summary>
        private void GetSlotsToMarkAsNotNullable(BoundExpression operand, ArrayBuilder<int> slotBuilder)
        {
            Debug.Assert(operand != null);
            var previousConditionalAccessSlot = _lastConditionalAccessSlot;

            try
            {
                while (true)
                {
                    // Due to the nature of binding, if there are conditional access they will be at the top of the bound tree,
                    // potentially with a conversion on top of it. We go through any conditional accesses, adding slots for the
                    // conditional receivers if they have them. If we ever get to a receiver that MakeSlot doesn't return a slot
                    // for, nothing underneath is trackable and we bail at that point. Example:
                    //
                    //     a?.GetB()?.C // a is a field, GetB is a method, and C is a property
                    //
                    // The top of the tree is the a?.GetB() conditional call. We'll ask for a slot for a, and we'll get one because
                    // fields have slots. The AccessExpression of the BoundConditionalAccess is another BoundConditionalAccess, this time
                    // with a receiver of the GetB() BoundCall. Attempting to get a slot for this receiver will fail, and we'll
                    // return an array with just the slot for a.
                    int slot;
                    switch (operand.Kind)
                    {
                        case BoundKind.Conversion:
                            // https://github.com/dotnet/roslyn/issues/33879 Detect when conversion has a nullable operand
                            operand = ((BoundConversion)operand).Operand;
                            continue;
                        case BoundKind.ConditionalAccess:
                            var conditional = (BoundConditionalAccess)operand;

                            slot = MakeSlot(conditional.Receiver);
                            if (slot > 0)
                            {
                                // We need to continue the walk regardless of whether the receiver should be updated.
                                var receiverType = conditional.Receiver.Type;
                                if (PossiblyNullableType(receiverType))
                                {
                                    slotBuilder.Add(slot);
                                }

                                if (receiverType.IsNullableType())
                                {
                                    slot = GetNullableOfTValueSlot(receiverType, slot, out _);
                                }
                            }

                            if (slot > 0)
                            {
                                // When MakeSlot is called on the nested AccessExpression, it will recurse through receivers
                                // until it gets to the BoundConditionalReceiver associated with this node. In our override,
                                // we substitute this slot when we encounter a BoundConditionalReceiver, and reset the
                                // _lastConditionalAccess field.
                                _lastConditionalAccessSlot = slot;
                                operand = conditional.AccessExpression;
                                continue;
                            }

                            // If there's no slot for this receiver, there cannot be another slot for any of the remaining
                            // access expressions.
                            break;
                        default:
                            // Attempt to create a slot for the current thing. If there were any more conditional accesses,
                            // they would have been on top, so this is the last thing we need to specially handle.

                            // https://github.com/dotnet/roslyn/issues/33879 When we handle unconditional access survival (ie after
                            // c.D has been invoked, c must be nonnull or we've thrown a NullRef), revisit whether
                            // we need more special handling here

                            slot = MakeSlot(operand);
                            if (slot > 0 && PossiblyNullableType(operand.Type))
                            {
                                slotBuilder.Add(slot);
                            }

                            break;
                    }

                    return;
                }
            }
            finally
            {
                _lastConditionalAccessSlot = previousConditionalAccessSlot;
            }
        }

        private static bool PossiblyNullableType(TypeSymbol operandType) => operandType?.CanContainNull() == true;

        private static void MarkSlotsAsNotNull(ArrayBuilder<int> slots, ref LocalState stateToUpdate)
        {
            if (slots is null)
            {
                return;
            }

            foreach (int slot in slots)
            {
                stateToUpdate[slot] = NullableFlowState.NotNull;
            }
        }

        private void LearnFromNonNullTest(BoundExpression expression, ref LocalState state)
        {
            var slotBuilder = ArrayBuilder<int>.GetInstance();
            GetSlotsToMarkAsNotNullable(expression, slotBuilder);
            MarkSlotsAsNotNull(slotBuilder, ref state);
            slotBuilder.Free();
        }

        private int LearnFromNullTest(BoundExpression expression, ref LocalState state)
        {
            var expressionWithoutConversion = RemoveConversion(expression, includeExplicitConversions: true).expression;
            var slot = MakeSlot(expressionWithoutConversion);
            if (slot > 0 && PossiblyNullableType(expressionWithoutConversion.Type))
            {
                SetStateAndTrackForFinally(ref state, slot, NullableFlowState.MaybeNull);
            }
            return slot;
        }

        private static BoundExpression SkipReferenceConversions(BoundExpression possiblyConversion)
        {
            while (possiblyConversion.Kind == BoundKind.Conversion)
            {
                var conversion = (BoundConversion)possiblyConversion;
                switch (conversion.ConversionKind)
                {
                    case ConversionKind.ImplicitReference:
                    case ConversionKind.ExplicitReference:
                        possiblyConversion = conversion.Operand;
                        break;
                    default:
                        return possiblyConversion;
                }
            }

            return possiblyConversion;
        }

        public override BoundNode VisitNullCoalescingAssignmentOperator(BoundNullCoalescingAssignmentOperator node)
        {
            BoundExpression leftOperand = node.LeftOperand;
            BoundExpression rightOperand = node.RightOperand;
            int leftSlot = MakeSlot(leftOperand);

            // The assignment to the left below needs the declared type from VisitLvalue, but the hidden
            // unnecessary check diagnostic needs the current adjusted type of the slot
            TypeWithAnnotations targetType = VisitLvalueWithAnnotations(leftOperand);
            var leftState = this.State.Clone();
            LearnFromNonNullTest(leftOperand, ref leftState);
            LearnFromNullTest(leftOperand, ref this.State);
            TypeWithState rightResult = VisitOptionalImplicitConversion(rightOperand, targetType, UseLegacyWarnings(leftOperand), AssignmentKind.Assignment);
            TrackNullableStateForAssignment(rightOperand, targetType, leftSlot, rightResult, MakeSlot(rightOperand));
            Join(ref this.State, ref leftState);
            TypeWithState resultType = GetNullCoalescingResultType(rightResult, targetType.Type);
            ResultType = resultType;
            return null;
        }

        public override BoundNode VisitNullCoalescingOperator(BoundNullCoalescingOperator node)
        {
            Debug.Assert(!IsConditionalState);

            BoundExpression leftOperand = node.LeftOperand;
            BoundExpression rightOperand = node.RightOperand;

            TypeWithState leftResult = VisitRvalueWithState(leftOperand);
            TypeWithState rightResult;

            if (IsConstantNull(leftOperand))
            {
                rightResult = VisitRvalueWithState(rightOperand);
                // Should be able to use rightResult for the result of the operator but
                // binding may have generated a different result type in the case of errors.
                ResultType = new TypeWithState(node.Type, rightResult.State);
                return null;
            }

            var whenNotNull = this.State.Clone();
            LearnFromNonNullTest(leftOperand, ref whenNotNull);
            LearnFromNullTest(leftOperand, ref this.State);

            bool leftIsConstant = leftOperand.ConstantValue != null;
            if (leftIsConstant)
            {
                SetUnreachable();
            }

            // https://github.com/dotnet/roslyn/issues/29955 For cases where the left operand determines
            // the type, we should unwrap the right conversion and re-apply.
            rightResult = VisitRvalueWithState(rightOperand);
            Join(ref this.State, ref whenNotNull);
            TypeSymbol resultType;
            var leftResultType = leftResult.Type;
            var rightResultType = rightResult.Type;
            switch (node.OperatorResultKind)
            {
                case BoundNullCoalescingOperatorResultKind.NoCommonType:
                    resultType = node.Type;
                    break;
                case BoundNullCoalescingOperatorResultKind.LeftType:
                    resultType = getLeftResultType(leftResultType, rightResultType);
                    break;
                case BoundNullCoalescingOperatorResultKind.LeftUnwrappedType:
                    resultType = getLeftResultType(leftResultType.StrippedType(), rightResultType);
                    break;
                case BoundNullCoalescingOperatorResultKind.RightType:
                    resultType = getRightResultType(leftResultType, rightResultType);
                    break;
                case BoundNullCoalescingOperatorResultKind.LeftUnwrappedRightType:
                    resultType = getRightResultType(leftResultType.StrippedType(), rightResultType);
                    break;
                case BoundNullCoalescingOperatorResultKind.RightDynamicType:
                    resultType = rightResultType;
                    break;
                default:
                    throw ExceptionUtilities.UnexpectedValue(node.OperatorResultKind);
            }

            ResultType = GetNullCoalescingResultType(rightResult, resultType);
            return null;

            TypeSymbol getLeftResultType(TypeSymbol leftType, TypeSymbol rightType)
            {
                Debug.Assert(!(rightType is null));
                // If there was an identity conversion between the two operands (in short, if there
                // is no implicit conversion on the right operand), then check nullable conversions
                // in both directions since it's possible the right operand is the better result type.
                if ((node.RightOperand as BoundConversion)?.ExplicitCastInCode != false &&
                    GenerateConversionForConditionalOperator(node.LeftOperand, leftType, rightType, reportMismatch: false).Exists)
                {
                    return rightType;
                }

                GenerateConversionForConditionalOperator(node.RightOperand, rightType, leftType, reportMismatch: true);
                return leftType;
            }

            TypeSymbol getRightResultType(TypeSymbol leftType, TypeSymbol rightType)
            {
                GenerateConversionForConditionalOperator(node.LeftOperand, leftType, rightType, reportMismatch: true);
                return rightType;
            }
        }

        /// <summary>
        /// Return top-level nullability for the expression. This method should be called on a limited
        /// set of expressions only. It should not be called on expressions tracked by flow analysis
        /// other than <see cref="BoundKind.ExpressionWithNullability"/> which is an expression
        /// specifically created in NullableWalker to represent the flow analysis state.
        /// </summary>
        private static NullableAnnotation GetNullableAnnotation(BoundExpression expr)
        {
            switch (expr.Kind)
            {
                case BoundKind.DefaultExpression:
                case BoundKind.Literal:
                    return (expr.ConstantValue?.IsNull != false) ? NullableAnnotation.NotAnnotated : NullableAnnotation.Annotated;
                case BoundKind.ExpressionWithNullability:
                    return ((BoundExpressionWithNullability)expr).NullableAnnotation;
                case BoundKind.MethodGroup:
                case BoundKind.UnboundLambda:
                    return NullableAnnotation.NotAnnotated;
                default:
                    Debug.Assert(false); // unexpected value
                    return NullableAnnotation.Oblivious;
            }
        }

        private static TypeWithState GetNullCoalescingResultType(TypeWithState rightResult, TypeSymbol resultType)
        {
            NullableFlowState resultState = rightResult.State;
            return new TypeWithState(resultType, resultState);
        }

        public override BoundNode VisitConditionalAccess(BoundConditionalAccess node)
        {
            Debug.Assert(!IsConditionalState);

            var receiver = node.Receiver;
            var receiverType = VisitRvalueWithState(receiver);
            _currentConditionalReceiverVisitResult = _visitResult;
            var previousConditionalAccessSlot = _lastConditionalAccessSlot;

            var receiverState = this.State.Clone();
            if (IsConstantNull(node.Receiver))
            {
                SetUnreachable();
                _lastConditionalAccessSlot = -1;
            }
            else
            {
                // In the when-null branch, the receiver is known to be maybe-null.
                // In the other branch, the receiver is known to be non-null.
                _lastConditionalAccessSlot = LearnFromNullTest(receiver, ref receiverState);
                LearnFromNonNullTest(receiver, ref this.State);
            }

            var accessExpressionType = VisitRvalueWithState(node.AccessExpression);
            Join(ref this.State, ref receiverState);
            // Per LDM 2019-02-13 decision, the result of a conditional access might be null even if
            // both the receiver and right-hand-side are believed not to be null.
            NullableFlowState resultState = NullableFlowState.MaybeNull;

            // https://github.com/dotnet/roslyn/issues/29956 Use flow analysis type rather than node.Type
            // so that nested nullability is inferred from flow analysis. See VisitConditionalOperator.
            TypeSymbol type = node.Type;

            // If the result type does not allow annotations, then we produce a warning because
            // the result may be null.
            if (RequiresSafetyWarningWhenNullIntroduced(type))
            {
                ReportSafetyDiagnostic(ErrorCode.WRN_ConditionalAccessMayReturnNull, node.Syntax, node.Type);
            }

            _currentConditionalReceiverVisitResult = default;
            _lastConditionalAccessSlot = previousConditionalAccessSlot;
            ResultType = new TypeWithState(type, resultState);
            return null;
        }

        public override BoundNode VisitConditionalOperator(BoundConditionalOperator node)
        {
            var isByRef = node.IsRef;

            VisitCondition(node.Condition);
            var consequenceState = this.StateWhenTrue;
            var alternativeState = this.StateWhenFalse;

            BoundExpression consequence;
            BoundExpression alternative;
            Conversion consequenceConversion;
            Conversion alternativeConversion;
            TypeWithAnnotations consequenceResult;
            TypeWithAnnotations alternativeResult;

            bool isConstantTrue = IsConstantTrue(node.Condition);
            bool isConstantFalse = IsConstantFalse(node.Condition);
            if (isConstantTrue)
            {
                (alternative, alternativeConversion, alternativeResult) = visitConditionalOperand(alternativeState, node.Alternative);
                (consequence, consequenceConversion, consequenceResult) = visitConditionalOperand(consequenceState, node.Consequence);
            }
            else if (isConstantFalse)
            {
                (consequence, consequenceConversion, consequenceResult) = visitConditionalOperand(consequenceState, node.Consequence);
                (alternative, alternativeConversion, alternativeResult) = visitConditionalOperand(alternativeState, node.Alternative);
            }
            else
            {
                (consequence, consequenceConversion, consequenceResult) = visitConditionalOperand(consequenceState, node.Consequence);
                Unsplit();
                consequenceState = this.State;
                (alternative, alternativeConversion, alternativeResult) = visitConditionalOperand(alternativeState, node.Alternative);
                Unsplit();
                Join(ref this.State, ref consequenceState);
            }

            TypeSymbol resultType;
            if (node.HasErrors)
            {
                resultType = null;
            }
            else
            {
                // Determine nested nullability using BestTypeInferrer.
                // For constant conditions, we could use the nested nullability of the particular
                // branch, but that requires using the nullability of the branch as it applies to the
                // target type. For instance, the result of the conditional in the following should
                // be `IEnumerable<object>` not `object[]`:
                //   object[] a = ...;
                //   IEnumerable<object?> b = ...;
                //   var c = true ? a : b;
                BoundExpression consequencePlaceholder = CreatePlaceholderIfNecessary(consequence, consequenceResult);
                BoundExpression alternativePlaceholder = CreatePlaceholderIfNecessary(alternative, alternativeResult);
                HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                // https://github.com/dotnet/roslyn/issues/30432: InferBestTypeForConditionalOperator should use node.IsRef.
                resultType = BestTypeInferrer.InferBestTypeForConditionalOperator(consequencePlaceholder, alternativePlaceholder, _conversions, out _, ref useSiteDiagnostics);
            }

            TypeWithAnnotations visitResult;
            if ((object)resultType != null)
            {
                var resultTypeWithAnnotations = TypeWithAnnotations.Create(resultType);
                TypeWithAnnotations convertedConsequenceResult = default;
                TypeWithAnnotations convertedAlternativeResult = default;

                if (!isConstantFalse)
                {
                    convertedConsequenceResult = convertResult(
                        node.Consequence,
                        consequence,
                        consequenceConversion,
                        resultTypeWithAnnotations,
                        consequenceResult);
                }

                if (!isConstantTrue)
                {
                    convertedAlternativeResult = convertResult(
                        node.Alternative,
                        alternative,
                        alternativeConversion,
                        resultTypeWithAnnotations,
                        alternativeResult);
                }

                if (!convertedAlternativeResult.HasType)
                {
                    Debug.Assert(convertedConsequenceResult.HasType);
                    visitResult = convertedConsequenceResult;
                }
                else if (!convertedConsequenceResult.HasType)
                {
                    Debug.Assert(convertedAlternativeResult.HasType);
                    visitResult = convertedAlternativeResult;
                }
                else
                {
                    visitResult = TypeWithAnnotations.Create(resultType,
                        convertedConsequenceResult.NullableAnnotation.Join(convertedAlternativeResult.NullableAnnotation));
                }
            }
            else
            {
                NullableAnnotation resultNullableAnnotation;

                if (isConstantTrue)
                {
                    resultNullableAnnotation = getNullableAnnotation(consequence, consequenceResult);
                }
                else if (isConstantFalse)
                {
                    resultNullableAnnotation = getNullableAnnotation(alternative, alternativeResult);
                }
                else
                {
                    resultNullableAnnotation = getNullableAnnotation(consequence, consequenceResult).Join(getNullableAnnotation(alternative, alternativeResult));
                }

                visitResult = TypeWithAnnotations.Create(node.Type.SetUnknownNullabilityForReferenceTypes(), resultNullableAnnotation);
            }
            LvalueResultType = visitResult;

            return null;

            NullableAnnotation getNullableAnnotation(BoundExpression expr, TypeWithAnnotations type)
            {
                if (type.HasType)
                {
                    return type.GetValueNullableAnnotation();
                }
                if (expr.IsLiteralNullOrDefault())
                {
                    return NullableAnnotation.Annotated;
                }
                return NullableAnnotation.Oblivious;
            }

            (BoundExpression, Conversion, TypeWithAnnotations) visitConditionalOperand(LocalState state, BoundExpression operand)
            {
                Conversion conversion;
                SetState(state);
                TypeWithAnnotations resultWithAnnotation;
                if (isByRef)
                {
                    resultWithAnnotation = VisitLvalueWithAnnotations(operand);
                    conversion = Conversion.Identity;
                }
                else
                {
                    (operand, conversion) = RemoveConversion(operand, includeExplicitConversions: false);
                    Visit(operand);
                    if (node.HasErrors)
                    {
                        ResultType = ResultType.WithNotNullState();
                    }

                    resultWithAnnotation = ResultType.ToTypeWithAnnotations();
                }

                return (operand, conversion, resultWithAnnotation);
            }

            TypeWithAnnotations convertResult(
                BoundExpression node,
                BoundExpression operand,
                Conversion conversion,
                TypeWithAnnotations targetType,
                TypeWithAnnotations operandType)
            {
                return ApplyConversion(
                    node,
                    operand,
                    conversion,
                    targetType,
                    operandType.ToTypeWithState(),
                    checkConversion: true,
                    fromExplicitCast: false,
                    useLegacyWarnings: false,
                    AssignmentKind.Assignment,
                    reportTopLevelWarnings: false).ToTypeWithAnnotations();
            }
        }

        /// <summary>
        /// Placeholders are bound expressions with type and state.
        /// But for typeless expressions (such as `null` or `(null, null)` we hold onto the original bound expression,
        /// as it will be useful for conversions from expression.
        /// </summary>
        private static BoundExpression CreatePlaceholderIfNecessary(BoundExpression expr, TypeWithAnnotations type)
        {
            return !type.HasType ?
                expr :
                new BoundExpressionWithNullability(expr.Syntax, expr, type.NullableAnnotation, type.Type);
        }

        public override BoundNode VisitConditionalReceiver(BoundConditionalReceiver node)
        {
            var rvalueType = _currentConditionalReceiverVisitResult.RValueType.Type;
            if (rvalueType?.IsNullableType() == true)
            {
                rvalueType = rvalueType.GetNullableUnderlyingType();
            }
            ResultType = new TypeWithState(rvalueType, NullableFlowState.NotNull);
            return null;
        }

        public override BoundNode VisitCall(BoundCall node)
        {
            // Note: we analyze even omitted calls
            TypeWithState receiverType = VisitCallReceiver(node);
            ReinferMethodAndVisitArguments(node, receiverType);
            return null;
        }

        private void ReinferMethodAndVisitArguments(BoundCall node, TypeWithState receiverType)
        {
            // https://github.com/dotnet/roslyn/issues/29605 Can we handle some error cases?
            // (Compare with CSharpOperationFactory.CreateBoundCallOperation.)
            var method = node.Method;
            ImmutableArray<RefKind> refKindsOpt = node.ArgumentRefKindsOpt;
            (ImmutableArray<BoundExpression> arguments, ImmutableArray<Conversion> conversions) = RemoveArgumentConversions(node.Arguments, refKindsOpt);
            if (!receiverType.HasNullType)
            {
                // Update method based on inferred receiver type.
                method = (MethodSymbol)AsMemberOfType(receiverType.Type, method);
            }

            method = VisitArguments(node, arguments, refKindsOpt, method.Parameters, node.ArgsToParamsOpt,
                node.Expanded, node.InvokedAsExtensionMethod, conversions, method).method;

            if (method.MethodKind == MethodKind.LocalFunction)
            {
                var localFunc = (LocalFunctionSymbol)method.OriginalDefinition;
                ReplayReadsAndWrites(localFunc, node.Syntax, writes: true);
            }

            var type = method.ReturnTypeWithAnnotations;
            LvalueResultType = type;
        }

        private TypeWithState VisitCallReceiver(BoundCall node)
        {
            var receiverOpt = node.ReceiverOpt;
            TypeWithState receiverType = default;

            if (receiverOpt != null && node.Method.MethodKind != MethodKind.Constructor)
            {
                receiverType = VisitRvalueWithState(receiverOpt);

                // methods which are members of Nullable<T> (ex: ToString, GetHashCode) can be invoked on null receiver.
                // However, inherited methods (ex: GetType) are invoked on a boxed value (since base types are reference types)
                // and therefore in those cases nullable receivers should be checked for nullness.
                bool checkNullableValueType = false;

                var type = receiverType.Type;
                var method = node.Method;
                if (!method.IsStatic &&
                    type?.IsNullableType() == true &&
                    method.ContainingType.IsReferenceType)
                {
                    checkNullableValueType = true;
                }
                else if (method.OriginalDefinition == compilation.GetSpecialTypeMember(SpecialMember.System_Nullable_T_get_Value))
                {
                    // call to get_Value may not occur directly in source, but may be inserted as a result of premature lowering. 
                    // One example where we do it is foreach with nullables. 
                    // The reason is Dev10 compatibility (see: UnwrapCollectionExpressionIfNullable in ForEachLoopBinder.cs)
                    // Regardless of the reasons, we know that the method does not tolerate nulls.
                    checkNullableValueType = true;
                }

                // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
                // after arguments have been visited, and only if the receiver has not changed.
                CheckPossibleNullReceiver(receiverOpt, checkNullableValueType);
            }

            return receiverType;
        }

        /// <summary>
        /// For each argument, figure out if its corresponding parameter is annotated with NotNullWhenFalse or
        /// EnsuresNotNull.
        /// </summary>
        private static ImmutableArray<FlowAnalysisAnnotations> GetAnnotations(int numArguments,
            bool expanded, ImmutableArray<ParameterSymbol> parameters, ImmutableArray<int> argsToParamsOpt)
        {
            ArrayBuilder<FlowAnalysisAnnotations> builder = null;

            for (int i = 0; i < numArguments; i++)
            {
                (ParameterSymbol parameter, _) = GetCorrespondingParameter(i, parameters, argsToParamsOpt, expanded);
                FlowAnalysisAnnotations annotations = parameter?.FlowAnalysisAnnotations ?? FlowAnalysisAnnotations.None;

                annotations = removeInapplicableAnnotations(parameter, annotations);

                if (annotations != FlowAnalysisAnnotations.None && builder == null)
                {
                    builder = ArrayBuilder<FlowAnalysisAnnotations>.GetInstance(numArguments);
                    builder.AddMany(FlowAnalysisAnnotations.None, i);
                }

                if (builder != null)
                {
                    builder.Add(annotations);
                }
            }

            return builder == null ? default : builder.ToImmutableAndFree();

            FlowAnalysisAnnotations removeInapplicableAnnotations(ParameterSymbol parameter, FlowAnalysisAnnotations annotations)
            {
                // Ignore NotNullWhenTrue that is inapplicable
                annotations = removeInapplicableNotNullWhenSense(parameter, annotations, sense: true);

                // Ignore NotNullWhenFalse that is inapplicable
                annotations = removeInapplicableNotNullWhenSense(parameter, annotations, sense: false);

                const FlowAnalysisAnnotations both = FlowAnalysisAnnotations.AssertsTrue | FlowAnalysisAnnotations.AssertsFalse;
                if (parameter?.Type.SpecialType != SpecialType.System_Boolean)
                {
                    // AssertsTrue and AssertsFalse must be applied to a bool parameter
                    annotations &= ~both;
                }
                else if ((annotations & both) == both)
                {
                    // We'll ignore AssertsTrue and AssertsFalse if both set
                    annotations &= ~both;
                }

                return annotations;
            }

            FlowAnalysisAnnotations removeInapplicableNotNullWhenSense(ParameterSymbol parameter, FlowAnalysisAnnotations annotations, bool sense)
            {
                if (parameter is null)
                {
                    return annotations;
                }

                var whenSense = sense ? FlowAnalysisAnnotations.NotNullWhenTrue : FlowAnalysisAnnotations.NotNullWhenFalse;
                var whenNotSense = sense ? FlowAnalysisAnnotations.NotNullWhenFalse : FlowAnalysisAnnotations.NotNullWhenTrue;

                // NotNullWhenSense (without NotNullWhenNotSense) must be applied on a bool-returning member
                if ((annotations & whenSense) != 0 &&
                    (annotations & whenNotSense) == 0 &&
                    parameter.ContainingSymbol.GetTypeOrReturnType().SpecialType != SpecialType.System_Boolean)
                {
                    annotations &= ~whenSense;
                }

                // NotNullWhenSense must be applied to a reference type, a nullable value type, or an unconstrained generic type
                if ((annotations & whenSense) != 0 && !parameter.Type.CanContainNull())
                {
                    annotations &= ~whenSense;
                }

                // NotNullWhenSense is inapplicable when argument corresponds to params parameter and we're in expanded form
                if ((annotations & whenSense) != 0 && expanded && ReferenceEquals(parameter, parameters.Last()))
                {
                    annotations &= ~whenSense;
                }

                return annotations;
            }
        }

        // https://github.com/dotnet/roslyn/issues/29863 Record in the node whether type
        // arguments were implicit, to allow for cases where the syntax is not an
        // invocation (such as a synthesized call from a query interpretation).
        private static bool HasImplicitTypeArguments(BoundExpression node)
        {
            var syntax = node.Syntax;
            if (syntax.Kind() != SyntaxKind.InvocationExpression)
            {
                // Unexpected syntax kind.
                return false;
            }
            var nameSyntax = Binder.GetNameSyntax(((InvocationExpressionSyntax)syntax).Expression, out var _);
            if (nameSyntax == null)
            {
                // Unexpected syntax kind.
                return false;
            }
            nameSyntax = nameSyntax.GetUnqualifiedName();
            return nameSyntax.Kind() != SyntaxKind.GenericName;
        }

        protected override void VisitArguments(ImmutableArray<BoundExpression> arguments, ImmutableArray<RefKind> refKindsOpt, MethodSymbol method)
        {
            // Callers should be using VisitArguments overload below.
            throw ExceptionUtilities.Unreachable;
        }

        private ImmutableArray<VisitResult> VisitArguments(
            BoundExpression node,
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<RefKind> refKindsOpt,
            MethodSymbol method,
            ImmutableArray<int> argsToParamsOpt,
            bool expanded)
        {
            ImmutableArray<Conversion> conversions;
            (arguments, conversions) = RemoveArgumentConversions(arguments, refKindsOpt);
            return VisitArguments(node, arguments, refKindsOpt, method is null ? default : method.Parameters, argsToParamsOpt, expanded, invokedAsExtensionMethod: false, conversions).results;
        }

        private ImmutableArray<VisitResult> VisitArguments(
            BoundExpression node,
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<RefKind> refKindsOpt,
            PropertySymbol property,
            ImmutableArray<int> argsToParamsOpt,
            bool expanded)
        {
            ImmutableArray<Conversion> conversions;
            (arguments, conversions) = RemoveArgumentConversions(arguments, refKindsOpt);
            return VisitArguments(node, arguments, refKindsOpt, property is null ? default : property.Parameters, argsToParamsOpt, expanded, invokedAsExtensionMethod: false, conversions).results;
        }

        /// <summary>
        /// If you pass in a method symbol, its type arguments will be re-inferred and the re-inferred method will be returned.
        /// </summary>
        private (MethodSymbol method, ImmutableArray<VisitResult> results) VisitArguments(
            BoundExpression node,
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<RefKind> refKindsOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<int> argsToParamsOpt,
            bool expanded,
            bool invokedAsExtensionMethod,
            ImmutableArray<Conversion> conversions,
            MethodSymbol method = null)
        {
            Debug.Assert(!arguments.IsDefault);
            var savedState = this.State.Clone();

            // We do a first pass to work through the arguments without making any assumptions
            ImmutableArray<VisitResult> results = VisitArgumentsEvaluate(arguments, refKindsOpt);

            if ((object)method != null && method.IsGenericMethod)
            {
                if (HasImplicitTypeArguments(node))
                {
                    method = InferMethodTypeArguments((BoundCall)node, method, GetArgumentsForMethodTypeInference(arguments, results));
                    parameters = method.Parameters;
                }
                if (ConstraintsHelper.RequiresChecking(method))
                {
                    var syntax = node.Syntax;
                    CheckMethodConstraints((syntax as InvocationExpressionSyntax)?.Expression ?? syntax, method);
                }
            }

            if (!node.HasErrors && !parameters.IsDefault)
            {
                VisitArgumentConversions(arguments, conversions, refKindsOpt, parameters, argsToParamsOpt, expanded, invokedAsExtensionMethod, results);
            }

            // We do a second pass through the arguments, ignoring any diagnostics produced, but honoring the annotations,
            // to get the proper result state.
            ImmutableArray<FlowAnalysisAnnotations> annotations = GetAnnotations(arguments.Length, expanded, parameters, argsToParamsOpt);

            if (!annotations.IsDefault)
            {
                this.SetState(savedState);

                bool saveDisableDiagnostics = _disableDiagnostics;
                _disableDiagnostics = true;
                if (!node.HasErrors && !parameters.IsDefault)
                {
                    // recompute out vars after state was reset
                    VisitArgumentConversions(arguments, conversions, refKindsOpt, parameters, argsToParamsOpt, expanded, invokedAsExtensionMethod, results);
                }
                VisitArgumentsEvaluateHonoringAnnotations(arguments, refKindsOpt, annotations);

                _disableDiagnostics = saveDisableDiagnostics;
            }

            return (method, results);
        }

        private ImmutableArray<VisitResult> VisitArgumentsEvaluate(ImmutableArray<BoundExpression> arguments, ImmutableArray<RefKind> refKindsOpt)
        {
            Debug.Assert(!IsConditionalState);
            int n = arguments.Length;
            if (n == 0)
            {
                return ImmutableArray<VisitResult>.Empty;
            }
            var builder = ArrayBuilder<VisitResult>.GetInstance(n);
            for (int i = 0; i < n; i++)
            {
                builder.Add(VisitArgumentEvaluate(arguments, refKindsOpt, i, preserveConditionalState: false));
            }

            ResultType = _invalidType;
            return builder.ToImmutableAndFree();
        }

        private VisitResult VisitArgumentEvaluate(ImmutableArray<BoundExpression> arguments, ImmutableArray<RefKind> refKindsOpt, int i, bool preserveConditionalState)
        {
            Debug.Assert(!IsConditionalState);
            RefKind refKind = GetRefKind(refKindsOpt, i);
            var argument = arguments[i];
            switch (refKind)
            {
                case RefKind.Ref:
                    Visit(argument);
                    if (!preserveConditionalState)
                    {
                        Unsplit();
                    }
                    break;
                case RefKind.None:
                case RefKind.In:
                    if (preserveConditionalState)
                    {
                        Visit(argument);
                        // No Unsplit
                        UseRvalueOnly(); // force use of flow result
                    }
                    else
                    {
                        VisitRvalue(argument);
                    }
                    break;
                case RefKind.Out:
                    // As far as we can tell, there is no scenario relevant to nullability analysis
                    // where splitting an L-value (for instance with a ref conditional) would affect the result.
                    Visit(argument);

                    // We'll want to use the l-value type, rather than the result type, for method re-inference
                    UseLvalueOnly();
                    break;
            }

            return _visitResult;
        }

        /// <summary>
        /// Visit all the arguments for the purpose of computing the exit state of the method,
        /// given the annotations.
        /// If there is any [NotNullWhenTrue/False] annotation, then we'll return in a conditional state for the invocation.
        /// </summary>
        private void VisitArgumentsEvaluateHonoringAnnotations(
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<RefKind> refKindsOpt,
            ImmutableArray<FlowAnalysisAnnotations> annotations)
        {
            Debug.Assert(!IsConditionalState);
            Debug.Assert(annotations.Length == arguments.Length);
            Debug.Assert(_disableDiagnostics);

            for (int i = 0; i < arguments.Length; i++)
            {
                FlowAnalysisAnnotations annotation = annotations[i];
                bool assertsTrue = (annotation & FlowAnalysisAnnotations.AssertsTrue) != 0;
                bool assertsFalse = (annotation & FlowAnalysisAnnotations.AssertsFalse) != 0;

                if (this.IsConditionalState)
                {
                    // We could be in a conditional state because of a conditional annotation (like NotNullWhenFalse)
                    // Then WhenTrue/False states correspond to the invocation returning true/false

                    // We'll first assume that we're in the unconditional state where the method returns true,
                    // then we'll repeat assuming the method returns false.

                    LocalState whenTrue = this.StateWhenTrue.Clone();
                    LocalState whenFalse = this.StateWhenFalse.Clone();

                    this.SetState(whenTrue);
                    visitArgumentEvaluateAndUnsplit(i, assertsTrue, assertsFalse);
                    Debug.Assert(!IsConditionalState);
                    whenTrue = this.State; // LocalState may be a struct

                    this.SetState(whenFalse);
                    visitArgumentEvaluateAndUnsplit(i, assertsTrue, assertsFalse);
                    Debug.Assert(!IsConditionalState);
                    whenFalse = this.State; // LocalState may be a struct

                    this.SetConditionalState(whenTrue, whenFalse);
                }
                else
                {
                    visitArgumentEvaluateAndUnsplit(i, assertsTrue, assertsFalse);
                }

                var argument = arguments[i];
                var argumentType = argument.Type;
                if (!PossiblyNullableType(argumentType))
                {
                    continue;
                }

                bool notNullWhenTrue = (annotation & FlowAnalysisAnnotations.NotNullWhenTrue) != 0;
                bool notNullWhenFalse = (annotation & FlowAnalysisAnnotations.NotNullWhenFalse) != 0;
                if (notNullWhenTrue || notNullWhenFalse)
                {
                    // The WhenTrue/False states correspond to the invocation returning true/false
                    bool wasPreviouslySplit = this.IsConditionalState;
                    Split();

                    var slotBuilder = ArrayBuilder<int>.GetInstance();
                    GetSlotsToMarkAsNotNullable(arguments[i], slotBuilder);

                    if (notNullWhenTrue)
                    {
                        MarkSlotsAsNotNull(slotBuilder, ref StateWhenTrue);
                    }
                    if (notNullWhenFalse)
                    {
                        MarkSlotsAsNotNull(slotBuilder, ref StateWhenFalse);
                        if (notNullWhenTrue && !wasPreviouslySplit) Unsplit();
                    }
                    slotBuilder.Free();
                }
            }

            ResultType = _invalidType;

            // Evaluate an argument, potentially producing a split state.
            // Then unsplit it based on [AssertsTrue] or [AssertsFalse] attributes, or default Unsplit otherwise.
            void visitArgumentEvaluateAndUnsplit(int argumentIndex, bool assertsTrue, bool assertsFalse)
            {
                Debug.Assert(!IsConditionalState);
                VisitArgumentEvaluate(arguments, refKindsOpt, argumentIndex, preserveConditionalState: true);

                if (!this.IsConditionalState)
                {
                    return;
                }
                else if (assertsTrue)
                {
                    this.SetState(this.StateWhenTrue);
                }
                else if (assertsFalse)
                {
                    this.SetState(this.StateWhenFalse);
                }
                else
                {
                    this.Unsplit();
                }
            }
        }

        private void VisitArgumentConversions(
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<Conversion> conversions,
            ImmutableArray<RefKind> refKindsOpt,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<int> argsToParamsOpt,
            bool expanded,
            bool invokedAsExtensionMethod,
            ImmutableArray<VisitResult> results)
        {
            for (int i = 0; i < arguments.Length; i++)
            {
                (ParameterSymbol parameter, TypeWithAnnotations parameterType) = GetCorrespondingParameter(i, parameters, argsToParamsOpt, expanded);
                if (parameter is null)
                {
                    continue;
                }
                VisitArgumentConversion(
                    arguments[i],
                    conversions.IsDefault ? Conversion.Identity : conversions[i],
                    GetRefKind(refKindsOpt, i),
                    parameter,
                    parameterType,
                    results[i],
                    invokedAsExtensionMethod && i == 0);
            }
        }

        /// <summary>
        /// Report warnings for an argument corresponding to a specific parameter.
        /// </summary>
        private void VisitArgumentConversion(
            BoundExpression argument,
            Conversion conversion,
            RefKind refKind,
            ParameterSymbol parameter,
            TypeWithAnnotations parameterType,
            VisitResult result,
            bool extensionMethodThisArgument)
        {
            // Note: we allow for some variance in `in` and `out` cases. Unlike in binding, we're not
            // limited by CLR constraints.

            var resultType = result.RValueType;
            bool reported = false;
            switch (refKind)
            {
                case RefKind.None:
                case RefKind.In:
                    {
                        ApplyConversion(
                            node: argument,
                            operandOpt: argument,
                            conversion: conversion,
                            targetTypeWithNullability: parameterType,
                            operandType: resultType,
                            checkConversion: true,
                            fromExplicitCast: false,
                            useLegacyWarnings: false,
                            assignmentKind: AssignmentKind.Argument,
                            target: parameter,
                            extensionMethodThisArgument: extensionMethodThisArgument);
                    }
                    break;
                case RefKind.Ref:
                    {
                        if (!argument.IsSuppressed)
                        {
                            var lvalueResultType = result.LValueType;
                            if (IsNullabilityMismatch(lvalueResultType, parameterType))
                            {
                                // declared types must match
                                ReportNullabilityMismatchInRefArgument(argument, argumentType: lvalueResultType, parameter, parameterType);
                            }
                            else
                            {
                                // types match, but state would let a null in
                                ReportNullableAssignmentIfNecessary(argument, parameterType, resultType, useLegacyWarnings: false);
                            }
                        }

                        // Check assignment from a fictional value from the parameter to the argument.
                        var parameterWithState = parameterType.ToTypeWithState();
                        if (argument.IsSuppressed)
                        {
                            parameterWithState = parameterWithState.WithNotNullState();
                        }

                        var parameterValue = new BoundParameter(argument.Syntax, parameter);
                        var lValueType = result.LValueType;
                        TrackNullableStateForAssignment(parameterValue, lValueType, MakeSlot(argument), parameterWithState);
                    }
                    break;
                case RefKind.Out:
                    {
                        var parameterWithState = parameterType.ToTypeWithState();
                        if (argument is BoundLocal local && local.DeclarationKind == BoundLocalDeclarationKind.WithInferredType)
                        {
                            _variableTypes[local.LocalSymbol] = parameterType;
                        }

                        var lValueType = result.LValueType;
                        // Check assignment from a fictional value from the parameter to the argument.
                        var parameterValue = new BoundParameter(argument.Syntax, parameter);

                        if (!argument.IsSuppressed && !reported)
                        {
                            ReportNullableAssignmentIfNecessary(parameterValue, lValueType, parameterWithState, useLegacyWarnings: UseLegacyWarnings(argument));

                            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                            if (!_conversions.HasIdentityOrImplicitReferenceConversion(parameterType.Type, lValueType.Type, ref useSiteDiagnostics))
                            {
                                ReportNullabilityMismatchInArgument(argument, lValueType.Type, parameter, parameterType.Type, forOutput: true);
                            }
                        }
                        else
                        {
                            parameterWithState = parameterWithState.WithNotNullState();
                        }

                        // Set nullable state of argument to parameter type.
                        TrackNullableStateForAssignment(parameterValue, lValueType, MakeSlot(argument), parameterWithState);
                    }
                    break;
                default:
                    throw ExceptionUtilities.UnexpectedValue(refKind);
            }
        }

        private static (ImmutableArray<BoundExpression> arguments, ImmutableArray<Conversion> conversions) RemoveArgumentConversions(
            ImmutableArray<BoundExpression> arguments,
            ImmutableArray<RefKind> refKindsOpt)
        {
            int n = arguments.Length;
            var conversions = default(ImmutableArray<Conversion>);
            if (n > 0)
            {
                var argumentsBuilder = ArrayBuilder<BoundExpression>.GetInstance(n);
                var conversionsBuilder = ArrayBuilder<Conversion>.GetInstance(n);
                bool includedConversion = false;
                for (int i = 0; i < n; i++)
                {
                    RefKind refKind = GetRefKind(refKindsOpt, i);
                    var argument = arguments[i];
                    var conversion = Conversion.Identity;
                    if (refKind == RefKind.None)
                    {
                        var before = argument;
                        (argument, conversion) = RemoveConversion(argument, includeExplicitConversions: false);
                        if (argument != before)
                        {
                            includedConversion = true;
                        }
                    }
                    argumentsBuilder.Add(argument);
                    conversionsBuilder.Add(conversion);
                }
                if (includedConversion)
                {
                    arguments = argumentsBuilder.ToImmutable();
                    conversions = conversionsBuilder.ToImmutable();
                }
                argumentsBuilder.Free();
                conversionsBuilder.Free();
            }
            return (arguments, conversions);
        }

        private VariableState GetVariableState()
        {
            // https://github.com/dotnet/roslyn/issues/29617 To track nullability of captured variables inside and
            // outside a lambda, the lambda should be considered executed at the location the lambda
            // is converted to a delegate.
            return new VariableState(
                _variableSlot.ToImmutableDictionary(),
                ImmutableArray.Create(variableBySlot, start: 0, length: nextVariableSlot),
                _variableTypes.ToImmutableDictionary());
        }

        private UnboundLambda GetUnboundLambda(BoundLambda expr, VariableState variableState)
        {
            return expr.UnboundLambda.WithNullableState(_binder, variableState);
        }

        private static (ParameterSymbol Parameter, TypeWithAnnotations Type) GetCorrespondingParameter(
            int argumentOrdinal,
            ImmutableArray<ParameterSymbol> parameters,
            ImmutableArray<int> argsToParamsOpt,
            bool expanded)
        {
            if (parameters.IsDefault)
            {
                return (default, default);
            }

            int n = parameters.Length;
            ParameterSymbol parameter;

            if (argsToParamsOpt.IsDefault)
            {
                if (argumentOrdinal < n)
                {
                    parameter = parameters[argumentOrdinal];
                }
                else if (expanded)
                {
                    parameter = parameters[n - 1];
                }
                else
                {
                    parameter = null;
                }
            }
            else
            {
                int parameterOrdinal = argsToParamsOpt[argumentOrdinal];

                if (parameterOrdinal < n)
                {
                    parameter = parameters[parameterOrdinal];
                }
                else
                {
                    parameter = null;
                    expanded = false;
                }
            }

            if (parameter is null)
            {
                Debug.Assert(!expanded);
                return (default, default);
            }

            var type = parameter.TypeWithAnnotations;
            if (expanded && parameter.Ordinal == n - 1 && type.IsSZArray())
            {
                type = ((ArrayTypeSymbol)type.Type).ElementTypeWithAnnotations;
            }

            return (parameter, type);
        }

        private MethodSymbol InferMethodTypeArguments(BoundCall node, MethodSymbol method, ImmutableArray<BoundExpression> arguments)
        {
            Debug.Assert(method.IsGenericMethod);

            // https://github.com/dotnet/roslyn/issues/27961 OverloadResolution.IsMemberApplicableInNormalForm and
            // IsMemberApplicableInExpandedForm use the least overridden method. We need to do the same here.
            var definition = method.ConstructedFrom;
            var refKinds = ArrayBuilder<RefKind>.GetInstance();
            if (node.ArgumentRefKindsOpt != null)
            {
                refKinds.AddRange(node.ArgumentRefKindsOpt);
            }

            // https://github.com/dotnet/roslyn/issues/27961 Do we really need OverloadResolution.GetEffectiveParameterTypes?
            // Aren't we doing roughly the same calculations in GetCorrespondingParameter?
            OverloadResolution.GetEffectiveParameterTypes(
                definition,
                arguments.Length,
                node.ArgsToParamsOpt,
                refKinds,
                isMethodGroupConversion: false,
                // https://github.com/dotnet/roslyn/issues/27961 `allowRefOmittedArguments` should be
                // false for constructors and several other cases (see Binder use). Should we
                // capture the original value in the BoundCall?
                allowRefOmittedArguments: true,
                binder: _binder,
                expanded: node.Expanded,
                parameterTypes: out ImmutableArray<TypeWithAnnotations> parameterTypes,
                parameterRefKinds: out ImmutableArray<RefKind> parameterRefKinds);

            refKinds.Free();
            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            var result = MethodTypeInferrer.Infer(
                _binder,
                _conversions,
                definition.TypeParameters,
                definition.ContainingType,
                parameterTypes,
                parameterRefKinds,
                arguments,
                ref useSiteDiagnostics,
                getTypeWithAnnotationOpt: s_getTypeWithAnnotations);

            if (!result.Success)
            {
                return method;
            }

            return definition.Construct(result.InferredTypeArguments);
        }

        private readonly static Func<BoundExpression, TypeWithAnnotations> s_getTypeWithAnnotations =
            (expr) => TypeWithAnnotations.Create(expr.Type, GetNullableAnnotation(expr));

        private ImmutableArray<BoundExpression> GetArgumentsForMethodTypeInference(ImmutableArray<BoundExpression> arguments, ImmutableArray<VisitResult> argumentResults)
        {
            // https://github.com/dotnet/roslyn/issues/27961 MethodTypeInferrer.Infer relies
            // on the BoundExpressions for tuple element types and method groups.
            // By using a generic BoundValuePlaceholder, we're losing inference in those cases.
            // https://github.com/dotnet/roslyn/issues/27961 Inference should be based on
            // unconverted arguments. Consider cases such as `default`, lambdas, tuples.
            int n = arguments.Length;
            var builder = ArrayBuilder<BoundExpression>.GetInstance(n);
            for (int i = 0; i < n; i++)
            {
                var argumentResult = argumentResults[i].LValueType;
                if (!argumentResult.HasType)
                    argumentResult = argumentResults[i].RValueType.ToTypeWithAnnotations();
                builder.Add(getArgumentForMethodTypeInference(arguments[i], argumentResult));
            }
            return builder.ToImmutableAndFree();

            BoundExpression getArgumentForMethodTypeInference(BoundExpression argument, TypeWithAnnotations argumentType)
            {
                if (argument.Kind == BoundKind.Lambda)
                {
                    // MethodTypeInferrer must infer nullability for lambdas based on the nullability
                    // from flow analysis rather than the declared nullability. To allow that, we need
                    // to re-bind lambdas in MethodTypeInferrer.
                    return GetUnboundLambda((BoundLambda)argument, GetVariableState());
                }
                if (!argumentType.HasType)
                {
                    return argument;
                }
                if (argument is BoundLocal local && local.DeclarationKind == BoundLocalDeclarationKind.WithInferredType)
                {
                    // 'out var' doesn't contribute to inference
                    return new BoundExpressionWithNullability(argument.Syntax, argument, NullableAnnotation.Oblivious, type: null);
                }
                return new BoundExpressionWithNullability(argument.Syntax, argument, argumentType.NullableAnnotation, argumentType.Type);
            }
        }

        private void CheckMethodConstraints(SyntaxNode syntax, MethodSymbol method)
        {
            var diagnosticsBuilder = ArrayBuilder<TypeParameterDiagnosticInfo>.GetInstance();
            var nullabilityBuilder = ArrayBuilder<TypeParameterDiagnosticInfo>.GetInstance();
            ArrayBuilder<TypeParameterDiagnosticInfo> useSiteDiagnosticsBuilder = null;
            ConstraintsHelper.CheckMethodConstraints(
                method,
                _conversions,
                includeNullability: true,
                compilation,
                diagnosticsBuilder,
                nullabilityBuilder,
                ref useSiteDiagnosticsBuilder);
            foreach (var pair in nullabilityBuilder)
            {
                Diagnostics.Add(pair.DiagnosticInfo, syntax.Location);
            }
            useSiteDiagnosticsBuilder?.Free();
            nullabilityBuilder.Free();
            diagnosticsBuilder.Free();
        }

        private void ReplayReadsAndWrites(LocalFunctionSymbol localFunc,
                                  SyntaxNode syntax,
                                  bool writes)
        {
            // https://github.com/dotnet/roslyn/issues/27233 Support field initializers in local functions.
        }

        /// <summary>
        /// Returns the expression without the top-most conversion plus the conversion.
        /// If the expression is not a conversion, returns the original expression plus
        /// the Identity conversion. If `includeExplicitConversions` is true, implicit and
        /// explicit conversions are considered. If `includeExplicitConversions` is false
        /// only implicit conversions are considered and if the expression is an explicit
        /// conversion, the expression is returned as is, with the Identity conversion.
        /// (Currently, the only visit method that passes `includeExplicitConversions: true`
        /// is VisitConversion. All other callers are handling implicit conversions only.)
        /// </summary>
        private static (BoundExpression expression, Conversion conversion) RemoveConversion(BoundExpression expr, bool includeExplicitConversions)
        {
            ConversionGroup group = null;
            while (true)
            {
                if (expr.Kind != BoundKind.Conversion)
                {
                    break;
                }
                var conversion = (BoundConversion)expr;
                if (group != conversion.ConversionGroupOpt && group != null)
                {
                    // E.g.: (C)(B)a
                    break;
                }
                group = conversion.ConversionGroupOpt;
                Debug.Assert(group != null || !conversion.ExplicitCastInCode); // Explicit conversions should include a group.
                if (!includeExplicitConversions && group?.IsExplicitConversion == true)
                {
                    return (expr, Conversion.Identity);
                }
                expr = conversion.Operand;
                if (group == null)
                {
                    // Ungrouped conversion should not be followed by another ungrouped
                    // conversion. Otherwise, the conversions should have been grouped.
                    Debug.Assert(expr.Kind != BoundKind.Conversion ||
                        ((BoundConversion)expr).ConversionGroupOpt != null ||
                        ((BoundConversion)expr).ConversionKind == ConversionKind.NoConversion);
                    return (expr, conversion.Conversion);
                }
            }
            return (expr, group?.Conversion ?? Conversion.Identity);
        }

        // See Binder.BindNullCoalescingOperator for initial binding.
        private Conversion GenerateConversionForConditionalOperator(BoundExpression sourceExpression, TypeSymbol sourceType, TypeSymbol destinationType, bool reportMismatch)
        {
            var conversion = GenerateConversion(_conversions, sourceExpression, sourceType, destinationType, fromExplicitCast: false, extensionMethodThisArgument: false);
            bool canConvertNestedNullability = conversion.Exists;
            if (!canConvertNestedNullability && reportMismatch && !sourceExpression.IsSuppressed)
            {
                ReportNullabilityMismatchInAssignment(sourceExpression.Syntax, GetTypeAsDiagnosticArgument(sourceType), destinationType);
            }
            return conversion;
        }

        private static Conversion GenerateConversion(Conversions conversions, BoundExpression sourceExpression, TypeSymbol sourceType, TypeSymbol destinationType, bool fromExplicitCast, bool extensionMethodThisArgument)
        {
            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            bool useExpression = UseExpressionForConversion(sourceExpression);
            if (extensionMethodThisArgument)
            {
                return conversions.ClassifyImplicitExtensionMethodThisArgConversion(
                    useExpression ? sourceExpression : null,
                    sourceType,
                    destinationType,
                    ref useSiteDiagnostics);
            }
            return useExpression ?
                (fromExplicitCast ?
                    conversions.ClassifyConversionFromExpression(sourceExpression, destinationType, ref useSiteDiagnostics, forCast: true) :
                    conversions.ClassifyImplicitConversionFromExpression(sourceExpression, destinationType, ref useSiteDiagnostics)) :
                (fromExplicitCast ?
                    conversions.ClassifyConversionFromType(sourceType, destinationType, ref useSiteDiagnostics, forCast: true) :
                    conversions.ClassifyImplicitConversionFromType(sourceType, destinationType, ref useSiteDiagnostics));
        }

        /// <summary>
        /// Returns true if the expression should be used as the source when calculating
        /// a conversion from this expression, rather than using the type (with nullability)
        /// calculated by visiting this expression. Typically, that means expressions that
        /// do not have an explicit type but there are several other cases as well.
        /// (See expressions handled in ClassifyImplicitBuiltInConversionFromExpression.)
        /// </summary>
        private static bool UseExpressionForConversion(BoundExpression value)
        {
            if (value is null)
            {
                return false;
            }
            if (value.Type is null || value.Type.IsDynamic() || value.ConstantValue != null)
            {
                return true;
            }
            switch (value.Kind)
            {
                case BoundKind.InterpolatedString:
                    return true;
                default:
                    return false;
            }
        }

        /// <summary>
        /// Adjust declared type based on inferred nullability at the point of reference.
        /// </summary>
        private TypeWithState GetAdjustedResult(TypeWithAnnotations type, int slot)
        {
            return GetAdjustedResult(type.ToTypeWithState(), slot);
        }

        private TypeWithState GetAdjustedResult(TypeWithState type, int slot)
        {
            if (slot > 0 && slot < this.State.Capacity)
            {
                NullableFlowState state = this.State[slot];
                return new TypeWithState(type.Type, state);
            }

            return type;
        }

        private static Symbol AsMemberOfType(TypeSymbol type, Symbol symbol)
        {
            Debug.Assert((object)symbol != null);

            var containingType = type as NamedTypeSymbol;
            if (containingType is null || containingType.IsErrorType() || symbol is ErrorMethodSymbol)
            {
                return symbol;
            }

            if (symbol.Kind == SymbolKind.Method)
            {
                if (((MethodSymbol)symbol).MethodKind == MethodKind.LocalFunction)
                {
                    // https://github.com/dotnet/roslyn/issues/27233 Handle type substitution for local functions.
                    return symbol;
                }
            }
            var symbolDef = symbol.OriginalDefinition;
            var symbolDefContainer = symbolDef.ContainingType;
            if (symbolDefContainer.IsTupleType)
            {
                return AsMemberOfTupleType((TupleTypeSymbol)containingType, symbol);
            }
            if (symbolDefContainer.IsAnonymousType)
            {
                int? memberIndex = symbol.Kind == SymbolKind.Property ? symbol.MemberIndexOpt : null;
                if (!memberIndex.HasValue)
                {
                    return symbol;
                }
                return AnonymousTypeManager.GetAnonymousTypeProperty(containingType, memberIndex.GetValueOrDefault());
            }
            if (!symbolDefContainer.IsGenericType)
            {
                Debug.Assert(symbol.ContainingType.IsDefinition);
                return symbol;
            }
            if (symbolDefContainer.IsInterface)
            {
                if (tryAsMemberOfSingleType(containingType, out Symbol result))
                {
                    return result;
                }
                foreach (var @interface in containingType.AllInterfacesNoUseSiteDiagnostics)
                {
                    if (tryAsMemberOfSingleType(@interface, out result))
                    {
                        return result;
                    }
                }
            }
            else
            {
                while (true)
                {
                    if (tryAsMemberOfSingleType(containingType, out Symbol result))
                    {
                        return result;
                    }
                    containingType = containingType.BaseTypeNoUseSiteDiagnostics;
                    if ((object)containingType == null)
                    {
                        break;
                    }
                }
            }
            Debug.Assert(false); // If this assert fails, add an appropriate test.
            return symbol;

            bool tryAsMemberOfSingleType(NamedTypeSymbol singleType, out Symbol result)
            {
                if (!singleType.OriginalDefinition.Equals(symbolDefContainer, TypeCompareKind.AllIgnoreOptions))
                {
                    result = null;
                    return false;
                }
                result = symbolDef.SymbolAsMember(singleType);
                if (result is MethodSymbol resultMethod && resultMethod.IsGenericMethod)
                {
                    result = resultMethod.Construct(((MethodSymbol)symbol).TypeArgumentsWithAnnotations);
                }
                return true;
            }
        }

        private static Symbol AsMemberOfTupleType(TupleTypeSymbol tupleType, Symbol symbol)
        {
            if (symbol.ContainingType.Equals(tupleType))
            {
                return symbol;
            }
            switch (symbol.Kind)
            {
                case SymbolKind.Field:
                    {
                        var index = ((FieldSymbol)symbol).TupleElementIndex;
                        if (index >= 0)
                        {
                            return tupleType.TupleElements[index];
                        }
                        return tupleType.GetTupleMemberSymbolForUnderlyingMember(((TupleFieldSymbol)symbol).UnderlyingField);
                    }
                case SymbolKind.Property:
                    return tupleType.GetTupleMemberSymbolForUnderlyingMember(((TuplePropertySymbol)symbol).UnderlyingProperty);
                case SymbolKind.Event:
                    return tupleType.GetTupleMemberSymbolForUnderlyingMember(((TupleEventSymbol)symbol).UnderlyingEvent);
                case SymbolKind.Method:
                    return tupleType.GetTupleMemberSymbolForUnderlyingMember(((TupleMethodSymbol)symbol).UnderlyingMethod);
                default:
                    throw ExceptionUtilities.UnexpectedValue(symbol.Kind);
            }
        }

        public override BoundNode VisitConversion(BoundConversion node)
        {
            // https://github.com/dotnet/roslyn/issues/29959 Assert VisitConversion is only used for explicit conversions.
            //Debug.Assert(node.ExplicitCastInCode);
            //Debug.Assert(node.ConversionGroupOpt != null);
            //Debug.Assert(!node.ConversionGroupOpt.ExplicitType.IsNull);

            TypeWithAnnotations explicitType = node.ConversionGroupOpt?.ExplicitType ?? default;
            bool fromExplicitCast = explicitType.HasType;
            TypeWithAnnotations targetType = fromExplicitCast ? explicitType : TypeWithAnnotations.Create(node.Type);
            Debug.Assert(targetType.HasType);

            (BoundExpression operand, Conversion conversion) = RemoveConversion(node, includeExplicitConversions: true);
            TypeWithState operandType = VisitRvalueWithState(operand);
            ResultType = ApplyConversion(
                node,
                operand,
                conversion,
                targetType,
                operandType,
                checkConversion: true,
                fromExplicitCast: fromExplicitCast,
                useLegacyWarnings: fromExplicitCast && !RequiresSafetyWarningWhenNullIntroduced(explicitType.Type),
                AssignmentKind.Assignment,
                reportTopLevelWarnings: fromExplicitCast,
                reportRemainingWarnings: true);

            switch (node.ConversionKind)
            {
                case ConversionKind.ImplicitNullable:
                case ConversionKind.ExplicitNullable:
                    TrackNullableStateIfNullableConversion(node);
                    break;
            }

            return null;
        }

        /// <summary>
        /// Visit an expression. If an explicit target type is provided, the expression is converted
        /// to that type. This method should be called whenever an expression may contain
        /// an implicit conversion, even if that conversion was omitted from the bound tree,
        /// so the conversion can be re-classified with nullability.
        /// </summary>
        private TypeWithState VisitOptionalImplicitConversion(BoundExpression expr, TypeWithAnnotations targetTypeOpt, bool useLegacyWarnings, AssignmentKind assignmentKind)
        {
            if (!targetTypeOpt.HasType)
            {
                return VisitRvalueWithState(expr);
            }

            (BoundExpression operand, Conversion conversion) = RemoveConversion(expr, includeExplicitConversions: false);
            var operandType = VisitRvalueWithState(operand);
            // If an explicit conversion was used in place of an implicit conversion, the explicit
            // conversion was created by initial binding after reporting "error CS0266:
            // Cannot implicitly convert type '...' to '...'. An explicit conversion exists ...".
            // Since an error was reported, we don't need to report nested warnings as well.
            bool reportNestedWarnings = !conversion.IsExplicit;
            var resultType = ApplyConversion(
                expr,
                operand,
                conversion,
                targetTypeOpt,
                operandType,
                checkConversion: true,
                fromExplicitCast: false,
                useLegacyWarnings: useLegacyWarnings,
                assignmentKind,
                reportTopLevelWarnings: true,
                reportRemainingWarnings: reportNestedWarnings);

            var conv = expr as BoundConversion;
            if (conv != null && conv.ConversionKind == ConversionKind.ImplicitNullable)
            {
                TrackNullableStateIfNullableConversion(conv);
            }

            return resultType;
        }

        private static bool AreNullableAndUnderlyingTypes(TypeSymbol nullableTypeOpt, TypeSymbol underlyingTypeOpt, out TypeWithAnnotations underlyingTypeWithAnnotations)
        {
            if (nullableTypeOpt?.IsNullableType() == true &&
                underlyingTypeOpt?.IsNullableType() == false)
            {
                var typeArg = nullableTypeOpt.GetNullableUnderlyingTypeWithAnnotations();
                if (typeArg.Type.Equals(underlyingTypeOpt, TypeCompareKind.AllIgnoreOptions))
                {
                    underlyingTypeWithAnnotations = typeArg;
                    return true;
                }
            }
            underlyingTypeWithAnnotations = default;
            return false;
        }

        public override BoundNode VisitTupleLiteral(BoundTupleLiteral node)
        {
            VisitTupleExpression(node);
            return null;
        }

        public override BoundNode VisitConvertedTupleLiteral(BoundConvertedTupleLiteral node)
        {
            VisitTupleExpression(node);
            return null;
        }

        private void VisitTupleExpression(BoundTupleExpression node)
        {
            var arguments = node.Arguments;
            ImmutableArray<TypeWithState> elementTypes = arguments.SelectAsArray((a, w) => w.VisitRvalueWithState(a), this);
            ImmutableArray<TypeWithAnnotations> elementTypesWithAnnotations = elementTypes.SelectAsArray(a => a.ToTypeWithAnnotations());
            var tupleOpt = (TupleTypeSymbol)node.Type;
            if (tupleOpt is null)
            {
                ResultType = default;
            }
            else
            {
                int slot = GetOrCreateObjectCreationPlaceholderSlot(node);
                if (slot > 0)
                {
                    this.State[slot] = NullableFlowState.NotNull;
                    TrackNullableStateOfTupleElements(slot, tupleOpt, arguments, elementTypes, useRestField: false);
                }

                tupleOpt = tupleOpt.WithElementTypes(elementTypesWithAnnotations);
                var locations = tupleOpt.TupleElements.SelectAsArray((element, location) => element.Locations.FirstOrDefault() ?? location, node.Syntax.Location);
                tupleOpt.CheckConstraints(_conversions, includeNullability: true, node.Syntax, locations, compilation, diagnosticsOpt: null, nullabilityDiagnosticsOpt: Diagnostics);
                ResultType = new TypeWithState(tupleOpt, NullableFlowState.NotNull);
            }
        }

        /// <summary>
        /// Set the nullability of tuple elements for tuples at the point of construction.
        /// If <paramref name="useRestField"/> is true, the tuple was constructed with an explicit
        /// 'new ValueTuple' call, in which case the 8-th element, if any, represents the 'Rest' field.
        /// </summary>
        private void TrackNullableStateOfTupleElements(
            int slot,
            TupleTypeSymbol tupleType,
            ImmutableArray<BoundExpression> values,
            ImmutableArray<TypeWithState> types,
            bool useRestField)
        {
            Debug.Assert(values.Length == types.Length);
            Debug.Assert(values.Length == (useRestField ? Math.Min(tupleType.TupleElements.Length, TupleTypeSymbol.RestPosition) : tupleType.TupleElements.Length));

            if (slot > 0)
            {
                var tupleElements = tupleType.TupleElements;
                int n = values.Length;
                if (useRestField)
                {
                    n = Math.Min(n, TupleTypeSymbol.RestPosition - 1);
                }
                for (int i = 0; i < n; i++)
                {
                    trackState(values[i], tupleElements[i], types[i]);
                }
                if (useRestField && values.Length == TupleTypeSymbol.RestPosition)
                {
                    var restField = tupleType.GetMembers(TupleTypeSymbol.RestFieldName).FirstOrDefault() as FieldSymbol;
                    if ((object)restField != null)
                    {
                        trackState(values.Last(), restField, types.Last());
                    }
                }
            }

            void trackState(BoundExpression value, FieldSymbol field, TypeWithState valueType) =>
                TrackNullableStateForAssignment(value, field.TypeWithAnnotations, GetOrCreateSlot(field, slot), valueType, MakeSlot(value));
        }

        private void TrackNullableStateOfNullableValue(int containingSlot, TypeSymbol containingType, BoundExpression value, TypeWithState valueType, int valueSlot)
        {
            Debug.Assert(containingType.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T);
            Debug.Assert(containingSlot > 0);
            Debug.Assert(valueSlot > 0);

            int targetSlot = GetNullableOfTValueSlot(containingType, containingSlot, out Symbol symbol);
            Debug.Assert(targetSlot > 0);
            if (targetSlot > 0)
            {
                TrackNullableStateForAssignment(value, symbol.GetTypeOrReturnType(), targetSlot, valueType, valueSlot);
            }
        }

        private void TrackNullableStateIfNullableConversion(BoundConversion node)
        {
            Debug.Assert(node.ConversionKind == ConversionKind.ImplicitNullable || node.ConversionKind == ConversionKind.ExplicitNullable);

            var operand = node.Operand;
            var operandType = operand.Type;
            var convertedType = node.Type;
            if (AreNullableAndUnderlyingTypes(convertedType, operandType, out TypeWithAnnotations underlyingType))
            {
                // Conversion of T to Nullable<T> is equivalent to new Nullable<T>(t).
                TrackNullableStateOfNullableValue(node, operand, convertedType, underlyingType);
            }
        }

        private void TrackNullableStateOfNullableValue(BoundExpression node, BoundExpression operand, TypeSymbol convertedType, TypeWithAnnotations underlyingType)
        {
            int valueSlot = MakeSlot(operand);
            if (valueSlot > 0)
            {
                int containingSlot = GetOrCreateObjectCreationPlaceholderSlot(node);
                Debug.Assert(containingSlot > 0);
                TrackNullableStateOfNullableValue(containingSlot, convertedType, operand, underlyingType.ToTypeWithState(), valueSlot);
            }
        }

        public override BoundNode VisitTupleBinaryOperator(BoundTupleBinaryOperator node)
        {
            base.VisitTupleBinaryOperator(node);
            SetNotNullResult(node);
            return null;
        }

        private void ReportNullabilityMismatchWithTargetDelegate(SyntaxNode syntax, NamedTypeSymbol delegateType, MethodSymbol method)
        {
            Debug.Assert((object)method != null);
            Debug.Assert(method.MethodKind != MethodKind.LambdaMethod);

            MethodSymbol invoke = delegateType?.DelegateInvokeMethod;
            if (invoke is null)
            {
                return;
            }

            if (IsNullabilityMismatch(method.ReturnTypeWithAnnotations, invoke.ReturnTypeWithAnnotations, requireIdentity: false))
            {
                ReportSafetyDiagnostic(ErrorCode.WRN_NullabilityMismatchInReturnTypeOfTargetDelegate, syntax,
                    new FormattedSymbol(method, SymbolDisplayFormat.MinimallyQualifiedFormat),
                    delegateType);
            }

            int count = Math.Min(invoke.ParameterCount, method.ParameterCount);
            for (int i = 0; i < count; i++)
            {
                var invokeParameter = invoke.Parameters[i];
                var methodParameter = method.Parameters[i];
                if (IsNullabilityMismatch(invokeParameter.TypeWithAnnotations, methodParameter.TypeWithAnnotations, requireIdentity: invokeParameter.RefKind != RefKind.None))
                {
                    ReportSafetyDiagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, syntax,
                        new FormattedSymbol(methodParameter, SymbolDisplayFormat.ShortFormat),
                        new FormattedSymbol(method, SymbolDisplayFormat.MinimallyQualifiedFormat),
                        delegateType);
                }
            }
        }

        private void ReportNullabilityMismatchWithTargetDelegate(SyntaxNode syntax, NamedTypeSymbol delegateType, UnboundLambda unboundLambda)
        {
            if (!unboundLambda.HasExplicitlyTypedParameterList)
            {
                return;
            }

            MethodSymbol invoke = delegateType?.DelegateInvokeMethod;
            if (invoke is null)
            {
                return;
            }

            int count = Math.Min(invoke.ParameterCount, unboundLambda.ParameterCount);
            for (int i = 0; i < count; i++)
            {
                var invokeParameter = invoke.Parameters[i];
                // Parameter nullability is expected to match exactly. This corresponds to the behavior of initial binding.
                //    Action<string> x = (object o) => { }; // error CS1661: Cannot convert lambda expression to delegate type 'Action<string>' because the parameter types do not match the delegate parameter types
                //    Action<object> y = (object? o) => { }; // warning CS8622: Nullability of reference types in type of parameter 'o' of 'lambda expression' doesn't match the target delegate 'Action<object>'.
                // https://github.com/dotnet/roslyn/issues/29959 Consider relaxing and allow implicit conversions of nullability.
                // (Compare with method group conversions which pass `requireIdentity: false`.)
                if (IsNullabilityMismatch(invokeParameter.TypeWithAnnotations, unboundLambda.ParameterTypeWithAnnotations(i), requireIdentity: true))
                {
                    // https://github.com/dotnet/roslyn/issues/29959 Consider using location of specific lambda parameter.
                    ReportSafetyDiagnostic(ErrorCode.WRN_NullabilityMismatchInParameterTypeOfTargetDelegate, syntax,
                        unboundLambda.ParameterName(i),
                        unboundLambda.MessageID.Localize(),
                        delegateType);
                }
            }
        }

        private bool IsNullabilityMismatch(TypeWithAnnotations source, TypeWithAnnotations destination, bool requireIdentity)
        {
            if (!HasTopLevelNullabilityConversion(source, destination, requireIdentity))
            {
                return true;
            }
            if (requireIdentity)
            {
                return IsNullabilityMismatch(source, destination);
            }
            var sourceType = source.Type;
            var destinationType = destination.Type;
            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            return !_conversions.ClassifyImplicitConversionFromType(sourceType, destinationType, ref useSiteDiagnostics).Exists;
        }

        private bool HasTopLevelNullabilityConversion(TypeWithAnnotations source, TypeWithAnnotations destination, bool requireIdentity)
        {
            return requireIdentity ?
                _conversions.HasTopLevelNullabilityIdentityConversion(source, destination) :
                _conversions.HasTopLevelNullabilityImplicitConversion(source, destination);
        }

        /// <summary>
        /// Apply the conversion to the type of the operand and return the resulting type. (If the
        /// operand does not have an explicit type, the operand expression is used for the type.)
        /// If `checkConversion` is set, the incoming conversion is assumed to be from binding and will be
        /// re-calculated, this time considering nullability. (Note that the conversion calculation considers
        /// nested nullability only. The caller is responsible for checking the top-level nullability of
        /// the type returned by this method.) `canConvertNestedNullability` is set if the conversion
        /// considering nested nullability succeeded. `node` is used only for the location of diagnostics.
        /// </summary>
        private TypeWithState ApplyConversion(
            BoundExpression node,
            BoundExpression operandOpt,
            Conversion conversion,
            TypeWithAnnotations targetTypeWithNullability,
            TypeWithState operandType,
            bool checkConversion,
            bool fromExplicitCast,
            bool useLegacyWarnings,
            AssignmentKind assignmentKind,
            ParameterSymbol target = null,
            bool reportTopLevelWarnings = true,
            bool reportRemainingWarnings = true,
            bool extensionMethodThisArgument = false)
        {
            Debug.Assert(node != null);
            Debug.Assert(operandOpt != null || !operandType.HasNullType);
            Debug.Assert(targetTypeWithNullability.HasType);
            Debug.Assert((object)target != null || assignmentKind != AssignmentKind.Argument);

            NullableFlowState resultState = NullableFlowState.NotNull;
            bool canConvertNestedNullability = true;
            bool isSuppressed = false;

            if (operandOpt?.IsSuppressed == true)
            {
                reportTopLevelWarnings = false;
                reportRemainingWarnings = false;
                isSuppressed = true;
            }

            TypeSymbol targetType = targetTypeWithNullability.Type;
            switch (conversion.Kind)
            {
                case ConversionKind.MethodGroup:
                    if (reportRemainingWarnings)
                    {
                        ReportNullabilityMismatchWithTargetDelegate(node.Syntax, targetType.GetDelegateType(), conversion.Method);
                    }
                    resultState = NullableFlowState.NotNull;
                    break;

                case ConversionKind.AnonymousFunction:
                    if (operandOpt.Kind == BoundKind.Lambda)
                    {
                        var lambda = (BoundLambda)operandOpt;
                        var delegateType = targetType.GetDelegateType();
                        var unboundLambda = lambda.UnboundLambda;
                        var variableState = GetVariableState();
                        Analyze(compilation, lambda, Diagnostics, delegateInvokeMethod: delegateType?.DelegateInvokeMethod, returnTypes: null, initialState: variableState);
                        if (reportRemainingWarnings)
                        {
                            ReportNullabilityMismatchWithTargetDelegate(node.Syntax, delegateType, unboundLambda);
                        }

                        return new TypeWithState(targetType, NullableFlowState.NotNull);
                    }
                    break;

                case ConversionKind.InterpolatedString:
                    resultState = NullableFlowState.NotNull;
                    break;

                case ConversionKind.ExplicitUserDefined:
                case ConversionKind.ImplicitUserDefined:
                    // cf. Binder.CreateUserDefinedConversion
                    {
                        if (!conversion.IsValid)
                        {
                            break;
                        }

                        // operand -> conversion "from" type
                        // May be distinct from method parameter type for Nullable<T>.
                        operandType = ApplyConversion(
                            node,
                            operandOpt,
                            conversion.UserDefinedFromConversion,
                            TypeWithAnnotations.Create(conversion.BestUserDefinedConversionAnalysis.FromType),
                            operandType,
                            checkConversion: true,
                            fromExplicitCast: false,
                            useLegacyWarnings,
                            assignmentKind,
                            target);

                        // Update method based on operandType: see https://github.com/dotnet/roslyn/issues/29605.
                        // (see NullableReferenceTypesTests.ImplicitConversions_07).
                        var methodOpt = conversion.Method;
                        Debug.Assert((object)methodOpt != null);
                        Debug.Assert(methodOpt.ParameterCount == 1);
                        var parameter = methodOpt.Parameters[0];
                        var parameterType = parameter.TypeWithAnnotations;
                        TypeWithState underlyingOperandType = default;
                        bool isLiftedConversion = false;
                        if (operandType.Type.IsNullableType() && !parameterType.IsNullableType())
                        {
                            var underlyingOperandTypeWithAnnotations = operandType.Type.GetNullableUnderlyingTypeWithAnnotations();
                            underlyingOperandType = underlyingOperandTypeWithAnnotations.ToTypeWithState();
                            isLiftedConversion = parameterType.Equals(underlyingOperandTypeWithAnnotations, TypeCompareKind.AllIgnoreOptions);
                        }

                        // conversion "from" type -> method parameter type
                        NullableFlowState operandState = operandType.State;
                        _ = ClassifyAndApplyConversion(operandOpt ?? node, parameterType, isLiftedConversion ? underlyingOperandType : operandType,
                            useLegacyWarnings, AssignmentKind.Argument, target: parameter, reportWarnings: reportRemainingWarnings);

                        // method parameter type -> method return type
                        var methodReturnType = methodOpt.ReturnTypeWithAnnotations;
                        if (isLiftedConversion)
                        {
                            operandType = LiftedReturnType(methodReturnType, operandState);
                            if (RequiresSafetyWarningWhenNullIntroduced(methodReturnType.Type) && operandState == NullableFlowState.MaybeNull)
                            {
                                ReportNullableAssignmentIfNecessary(node, targetTypeWithNullability, operandType, useLegacyWarnings: useLegacyWarnings, assignmentKind, target, conversion: conversion);
                            }
                        }
                        else
                        {
                            operandType = methodReturnType.ToTypeWithState();
                        }

                        // method return type -> conversion "to" type
                        // May be distinct from method return type for Nullable<T>.
                        operandType = ClassifyAndApplyConversion(operandOpt ?? node, TypeWithAnnotations.Create(conversion.BestUserDefinedConversionAnalysis.ToType), operandType,
                            useLegacyWarnings, assignmentKind, target, reportWarnings: reportRemainingWarnings);

                        // conversion "to" type -> final type
                        // https://github.com/dotnet/roslyn/issues/29959 If the original conversion was
                        // explicit, this conversion should not report nested nullability mismatches.
                        // (see NullableReferenceTypesTests.ExplicitCast_UserDefined_02).
                        operandType = ClassifyAndApplyConversion(node, targetTypeWithNullability, operandType,
                            useLegacyWarnings, assignmentKind, target, reportWarnings: reportRemainingWarnings);
                        return operandType;
                    }

                case ConversionKind.ExplicitDynamic:
                case ConversionKind.ImplicitDynamic:
                case ConversionKind.Unboxing:
                case ConversionKind.Boxing:
                    resultState = operandType.State;
                    break;

                case ConversionKind.ImplicitThrow:
                    break;

                case ConversionKind.NoConversion:
                case ConversionKind.DefaultOrNullLiteral:
                    checkConversion = false;
                    goto case ConversionKind.Identity;

                case ConversionKind.Identity:
                    // If the operand is an explicit conversion, and this identity conversion
                    // is converting to the same type including nullability, skip the conversion
                    // to avoid reporting redundant warnings. Also check useLegacyWarnings
                    // since that value was used when reporting warnings for the explicit cast.
                    if (useLegacyWarnings && operandOpt?.Kind == BoundKind.Conversion)
                    {
                        var operandConversion = (BoundConversion)operandOpt;
                        var explicitType = operandConversion.ConversionGroupOpt.ExplicitType;
                        if (explicitType.HasType && explicitType.Equals(targetTypeWithNullability, TypeCompareKind.ConsiderEverything))
                        {
                            return operandType;
                        }
                    }
                    if (operandType.Type?.IsTupleType == true)
                    {
                        goto case ConversionKind.ImplicitTuple;
                    }
                    goto case ConversionKind.ImplicitReference;

                case ConversionKind.ImplicitReference:
                    if (reportTopLevelWarnings &&
                        operandOpt?.Kind == BoundKind.Literal &&
                        operandOpt.ConstantValue?.IsNull == true &&
                        !isSuppressed &&
                        RequiresSafetyWarningWhenNullIntroduced(targetType))
                    {
                        // For type parameters that cannot be annotated, the analysis must report those
                        // places where null values first sneak in, like `default`, `null`, and `GetFirstOrDefault`.
                        // This is one of those places.
                        ReportSafetyDiagnostic(ErrorCode.WRN_NullLiteralMayIntroduceNullT, node.Syntax, targetType);
                    }
                    goto case ConversionKind.ExplicitReference;

                case ConversionKind.ExplicitReference:
                    // Inherit state from the operand.
                    if (checkConversion)
                    {
                        // https://github.com/dotnet/roslyn/issues/29959 Assert conversion is similar to original.
                        conversion = GenerateConversion(_conversions, operandOpt, operandType.Type, targetType, fromExplicitCast, extensionMethodThisArgument);
                        canConvertNestedNullability = conversion.Exists;
                    }

                    resultState = operandType.State;
                    break;

                case ConversionKind.ImplicitNullable:
                    if (checkConversion)
                    {
                        conversion = GenerateConversion(_conversions, operandOpt, operandType.Type, targetType, fromExplicitCast, extensionMethodThisArgument);
                        canConvertNestedNullability = conversion.Exists;
                    }

                    resultState = operandType.State;
                    break;

                case ConversionKind.ExplicitNullable:
                    if (operandType.Type?.IsNullableType() == true && !targetType.IsNullableType())
                    {
                        // Explicit conversion of Nullable<T> to T is equivalent to Nullable<T>.Value.
                        if (reportTopLevelWarnings && operandType.MayBeNull)
                        {
                            ReportSafetyDiagnostic(ErrorCode.WRN_NullableValueTypeMayBeNull, node.Syntax);
                        }

                        // Mark the value as not nullable, regardless of whether it was known to be nullable,
                        // because the implied call to `.Value` will only succeed if not null.
                        if (operandOpt != null)
                        {
                            int slot = MakeSlot(operandOpt);
                            if (slot > 0)
                            {
                                this.State[slot] = NullableFlowState.NotNull;
                            }
                        }
                    }
                    goto case ConversionKind.ImplicitNullable;

                case ConversionKind.ImplicitTupleLiteral:
                case ConversionKind.ImplicitTuple:
                case ConversionKind.ExplicitTupleLiteral:
                case ConversionKind.ExplicitTuple:
                    if (checkConversion)
                    {
                        // https://github.com/dotnet/roslyn/issues/29699: Report warnings for user-defined conversions on tuple elements.
                        conversion = GenerateConversion(_conversions, operandOpt, operandType.Type, targetType, fromExplicitCast, extensionMethodThisArgument);
                        canConvertNestedNullability = conversion.Exists;
                    }
                    resultState = NullableFlowState.NotNull;
                    break;

                case ConversionKind.Deconstruction:
                    // Can reach here, with an error type, when the
                    // Deconstruct method is missing or inaccessible.
                    break;

                case ConversionKind.ExplicitEnumeration:
                    // Can reach here, with an error type.
                    break;

                default:
                    Debug.Assert(targetType.IsValueType);
                    break;
            }

            if (isSuppressed)
            {
                resultState = NullableFlowState.NotNull;
            }
            else if (fromExplicitCast && targetTypeWithNullability.NullableAnnotation.IsAnnotated() && !targetType.IsNullableType())
            {
                // An explicit cast to a nullable reference type introduces nullability
                resultState = NullableFlowState.MaybeNull;
            }

            var resultType = new TypeWithState(targetType, resultState);

            if (operandType.Type?.IsErrorType() != true && !targetType.IsErrorType())
            {
                // Need to report all warnings that apply since the warnings can be suppressed individually.
                if (reportTopLevelWarnings)
                {
                    if (RequiresSafetyWarningWhenNullIntroduced(targetType) && conversion.IsImplicit && !conversion.IsDynamic)
                    {
                        // For type parameters that cannot be annotated, the analysis must report those
                        // places where null values first sneak in, like `default`, `null`, and `GetFirstOrDefault`,
                        // as a safety diagnostic.  But we do not warn when such values flow through implicit conversion.
                    }
                    else
                    {
                        ReportNullableAssignmentIfNecessary(node, targetTypeWithNullability, operandType, useLegacyWarnings, assignmentKind, target, conversion: conversion);
                    }
                }
                if (reportRemainingWarnings && !canConvertNestedNullability)
                {
                    if (assignmentKind == AssignmentKind.Argument)
                    {
                        ReportNullabilityMismatchInArgument(node, operandType.Type, target, targetType, forOutput: false);
                    }
                    else
                    {
                        ReportNullabilityMismatchInAssignment(node.Syntax, GetTypeAsDiagnosticArgument(operandType.Type), targetType);
                    }
                }
            }

            return resultType;
        }

        /// <summary>
        /// Return the return type for a lifted operator, given the nullability state of its operands.
        /// </summary>
        private TypeWithState LiftedReturnType(TypeWithAnnotations returnType, NullableFlowState operandState)
        {
            bool typeNeedsLifting = returnType.Type.IsNonNullableValueType();
            TypeSymbol type = typeNeedsLifting
                ? compilation.GetSpecialType(SpecialType.System_Nullable_T).Construct(ImmutableArray.Create(returnType))
                : returnType.Type;
            NullableFlowState state = returnType.ToTypeWithState().State.Join(operandState);
            return new TypeWithState(type, state);
        }

        private TypeWithState ClassifyAndApplyConversion(
            BoundExpression node,
            TypeWithAnnotations targetType,
            TypeWithState operandType,
            bool useLegacyWarnings,
            AssignmentKind assignmentKind,
            ParameterSymbol target,
            bool reportWarnings)
        {
            Debug.Assert((object)target != null || assignmentKind != AssignmentKind.Argument);
            HashSet<DiagnosticInfo> useSiteDiagnostics = null;
            var conversion = _conversions.ClassifyStandardConversion(null, operandType.Type, targetType.Type, ref useSiteDiagnostics);
            if (reportWarnings && !conversion.Exists)
            {
                if (assignmentKind == AssignmentKind.Argument)
                {
                    ReportNullabilityMismatchInArgument(node, operandType.Type, target, targetType.Type, forOutput: false);
                }
                else
                {
                    ReportNullabilityMismatchInAssignment(node.Syntax, operandType.Type, targetType.Type);
                }
            }

            return ApplyConversion(
                node,
                operandOpt: null,
                conversion,
                targetType,
                operandType,
                checkConversion: false,
                fromExplicitCast: false,
                useLegacyWarnings: useLegacyWarnings,
                assignmentKind,
                target,
                reportTopLevelWarnings: reportWarnings,
                reportRemainingWarnings: reportWarnings);
        }

        public override BoundNode VisitDelegateCreationExpression(BoundDelegateCreationExpression node)
        {
            if (node.MethodOpt?.MethodKind == MethodKind.LocalFunction)
            {
                var syntax = node.Syntax;
                var localFunc = (LocalFunctionSymbol)node.MethodOpt.OriginalDefinition;
                ReplayReadsAndWrites(localFunc, syntax, writes: false);
            }

            base.VisitDelegateCreationExpression(node);
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitMethodGroup(BoundMethodGroup node)
        {
            Debug.Assert(!IsConditionalState);

            BoundExpression receiverOpt = node.ReceiverOpt;
            if (receiverOpt != null)
            {
                VisitRvalue(receiverOpt);
                // https://github.com/dotnet/roslyn/issues/30563: Should not check receiver here.
                // That check should be handled when applying the method group conversion,
                // when we have a specific method, to avoid reporting null receiver warnings
                // for extension method delegates.
                CheckPossibleNullReceiver(receiverOpt);
            }

            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitLambda(BoundLambda node)
        {
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitUnboundLambda(UnboundLambda node)
        {
            // The presence of this node suggests an error was detected in an earlier phase.
            // Analyze the body to report any additional warnings.
            var lambda = node.BindForErrorRecovery();
            Analyze(compilation, lambda, Diagnostics, delegateInvokeMethod: null, returnTypes: null, initialState: GetVariableState());
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitLocalFunctionStatement(BoundLocalFunctionStatement node)
        {
            var body = node.Body;
            if (body != null)
            {
                Analyze(
                    compilation,
                    node.Symbol,
                    body,
                    Diagnostics,
                    useMethodSignatureReturnType: false,
                    useMethodSignatureParameterTypes: false,
                    methodSignatureOpt: null,
                    returnTypes: null,
                    initialState: GetVariableState(),
                    callbackOpt: _callbackOpt);
            }
            ResultType = _invalidType;
            return null;
        }

        public override BoundNode VisitThisReference(BoundThisReference node)
        {
            VisitThisOrBaseReference(node);
            return null;
        }

        private void VisitThisOrBaseReference(BoundExpression node)
        {
            var rvalueResult = new TypeWithState(node.Type, NullableFlowState.NotNull);
            var lvalueResult = TypeWithAnnotations.Create(node.Type, NullableAnnotation.NotAnnotated);
            SetResult(rvalueResult, lvalueResult);
        }

        public override BoundNode VisitParameter(BoundParameter node)
        {
            var parameter = node.ParameterSymbol;
            int slot = GetOrCreateSlot(parameter);
            var type = GetDeclaredParameterResult(parameter);
            SetResult(GetAdjustedResult(type, slot), type);
            return null;
        }

        public override BoundNode VisitAssignmentOperator(BoundAssignmentOperator node)
        {
            Debug.Assert(!IsConditionalState);

            var left = node.Left;
            var right = node.Right;
            Visit(left);
            TypeWithAnnotations leftLValueType = LvalueResultType;

            if (left.Kind == BoundKind.EventAccess && ((BoundEventAccess)left).EventSymbol.IsWindowsRuntimeEvent)
            {
                // Event assignment is a call to an Add method. (Note that assignment
                // of non-field-like events uses BoundEventAssignmentOperator
                // rather than BoundAssignmentOperator.)
                VisitRvalue(right);
                SetNotNullResult(node);
            }
            else
            {
                TypeWithState rightType;
                if (!node.IsRef)
                {
                    rightType = VisitOptionalImplicitConversion(right, leftLValueType, UseLegacyWarnings(left), AssignmentKind.Assignment);
                }
                else
                {
                    rightType = VisitRefExpression(right, leftLValueType);
                }

                TrackNullableStateForAssignment(right, leftLValueType, MakeSlot(left), rightType, MakeSlot(right));
                SetResult(new TypeWithState(leftLValueType.Type, rightType.State), leftLValueType);
            }

            return null;
        }

        private static bool UseLegacyWarnings(BoundExpression expr)
        {
            switch (expr.Kind)
            {
                case BoundKind.Local:
                    return !RequiresSafetyWarningWhenNullIntroduced(expr.Type);
                case BoundKind.Parameter:
                    RefKind kind = ((BoundParameter)expr).ParameterSymbol.RefKind;
                    return kind == RefKind.None && !RequiresSafetyWarningWhenNullIntroduced(expr.Type);
                default:
                    return false;
            }
        }

        public override BoundNode VisitDeconstructionAssignmentOperator(BoundDeconstructionAssignmentOperator node)
        {
            var left = node.Left;
            var right = node.Right;
            var variables = GetDeconstructionAssignmentVariables(left);

            if (node.HasErrors)
            {
                // In the case of errors, simply visit the right as an r-value to update
                // any nullability state even though deconstruction is skipped.
                VisitRvalue(right.Operand);
            }
            else
            {
                VisitDeconstructionArguments(variables, right.Conversion, right.Operand);
            }

            variables.FreeAll(v => v.NestedVariables);

            // https://github.com/dotnet/roslyn/issues/33011: Result type should be inferred and the constraints should
            // be re-verified. Even though the standard tuple type has no constraints we support that scenario. Constraints_78
            // has a test for this case that should start failing when this is fixed.
            SetNotNullResult(node);
            return null;
        }

        private void VisitDeconstructionArguments(ArrayBuilder<DeconstructionVariable> variables, Conversion conversion, BoundExpression right)
        {
            Debug.Assert(conversion.Kind == ConversionKind.Deconstruction);

            int n = variables.Count;

            if (!conversion.DeconstructionInfo.IsDefault)
            {
                VisitRvalue(right);

                var invocation = conversion.DeconstructionInfo.Invocation as BoundCall;
                var deconstructMethod = invocation?.Method;

                if ((object)deconstructMethod != null)
                {
                    if (!invocation.InvokedAsExtensionMethod)
                    {
                        CheckPossibleNullReceiver(right);
                    }
                    else
                    {
                        // https://github.com/dotnet/roslyn/issues/33006: Check nullability of `this` argument.
                    }

                    // https://github.com/dotnet/roslyn/issues/33006: Update `Deconstruct` method
                    // based on inferred receiver type, and check constraints.

                    var parameters = deconstructMethod.Parameters;
                    int offset = invocation.InvokedAsExtensionMethod ? 1 : 0;
                    Debug.Assert(parameters.Length - offset == n);

                    for (int i = 0; i < n; i++)
                    {
                        var variable = variables[i];
                        var underlyingConversion = conversion.UnderlyingConversions[i];
                        var nestedVariables = variable.NestedVariables;
                        if (nestedVariables != null)
                        {
                            // https://github.com/dotnet/roslyn/issues/33005: Not handling deconstructing argument of Deconstruct.
                            //VisitDeconstructionArguments(nestedVariables, underlyingConversion, arg);
                        }
                        else
                        {
                            var parameter = parameters[i + offset];
                            VisitArgumentConversion(
                                variable.Expression, underlyingConversion, parameter.RefKind, parameter, parameter.TypeWithAnnotations,
                                new VisitResult(variable.Type.ToTypeWithState(), variable.Type), extensionMethodThisArgument: false);
                        }
                    }
                }
            }
            else
            {
                var rightParts = GetDeconstructionRightParts(right);
                Debug.Assert(rightParts.Length == n);

                for (int i = 0; i < n; i++)
                {
                    var variable = variables[i];
                    var underlyingConversion = conversion.UnderlyingConversions[i];
                    var rightPart = rightParts[i];
                    var nestedVariables = variable.NestedVariables;
                    if (nestedVariables != null)
                    {
                        VisitDeconstructionArguments(nestedVariables, underlyingConversion, rightPart);
                    }
                    else
                    {
                        var targetType = variable.Type;
                        TypeWithState operandType;
                        TypeWithState valueType;
                        int valueSlot;
                        if (underlyingConversion.IsIdentity)
                        {
                            operandType = default;
                            valueType = VisitOptionalImplicitConversion(rightPart, targetType, useLegacyWarnings: true, AssignmentKind.Assignment);
                            valueSlot = MakeSlot(rightPart);
                        }
                        else
                        {
                            operandType = VisitRvalueWithState(rightPart);
                            valueType = ApplyConversion(
                                rightPart,
                                rightPart,
                                underlyingConversion,
                                targetType,
                                operandType,
                                checkConversion: true,
                                fromExplicitCast: false,
                                useLegacyWarnings: true,
                                AssignmentKind.Assignment,
                                reportTopLevelWarnings: true,
                                reportRemainingWarnings: true);
                            valueSlot = -1;
                        }

                        int targetSlot = MakeSlot(variable.Expression);
                        TrackNullableStateForAssignment(rightPart, targetType, targetSlot, valueType, valueSlot);

                        // Conversion of T to Nullable<T> is equivalent to new Nullable<T>(t).
                        // (Should this check be moved to VisitOptionalImplicitConversion or TrackNullableStateForAssignment?)
                        if (targetSlot > 0 &&
                            underlyingConversion.Kind == ConversionKind.ImplicitNullable &&
                            AreNullableAndUnderlyingTypes(targetType.Type, operandType.Type, out TypeWithAnnotations underlyingType))
                        {
                            valueSlot = MakeSlot(rightPart);
                            if (valueSlot > 0)
                            {
                                var valueBeforeNullableWrapping = new TypeWithState(underlyingType.Type, NullableFlowState.NotNull);
                                TrackNullableStateOfNullableValue(targetSlot, targetType.Type, rightPart, valueBeforeNullableWrapping, valueSlot);
                            }
                        }
                    }
                }
            }
        }

        private readonly struct DeconstructionVariable
        {
            internal readonly BoundExpression Expression;
            internal readonly TypeWithAnnotations Type;
            internal readonly ArrayBuilder<DeconstructionVariable> NestedVariables;

            internal DeconstructionVariable(BoundExpression expression, TypeWithAnnotations type)
            {
                Expression = expression;
                Type = type;
                NestedVariables = null;
            }

            internal DeconstructionVariable(ArrayBuilder<DeconstructionVariable> nestedVariables)
            {
                Expression = null;
                Type = default;
                NestedVariables = nestedVariables;
            }
        }

        private ArrayBuilder<DeconstructionVariable> GetDeconstructionAssignmentVariables(BoundTupleExpression tuple)
        {
            var arguments = tuple.Arguments;
            var builder = ArrayBuilder<DeconstructionVariable>.GetInstance(arguments.Length);
            foreach (var argument in arguments)
            {
                builder.Add(getDeconstructionAssignmentVariable(argument));
            }
            return builder;

            DeconstructionVariable getDeconstructionAssignmentVariable(BoundExpression expr)
            {
                switch (expr.Kind)
                {
                    case BoundKind.TupleLiteral:
                    case BoundKind.ConvertedTupleLiteral:
                        return new DeconstructionVariable(GetDeconstructionAssignmentVariables((BoundTupleExpression)expr));
                    default:
                        Visit(expr);
                        return new DeconstructionVariable(expr, LvalueResultType);
                }
            }
        }

        /// <summary>
        /// Return the sub-expressions for the righthand side of a deconstruction
        /// assignment. cf. LocalRewriter.GetRightParts.
        /// </summary>
        private ImmutableArray<BoundExpression> GetDeconstructionRightParts(BoundExpression expr)
        {
            switch (expr.Kind)
            {
                case BoundKind.TupleLiteral:
                case BoundKind.ConvertedTupleLiteral:
                    return ((BoundTupleExpression)expr).Arguments;
                case BoundKind.Conversion:
                    {
                        var conv = (BoundConversion)expr;
                        switch (conv.ConversionKind)
                        {
                            case ConversionKind.Identity:
                            case ConversionKind.ImplicitTupleLiteral:
                                return GetDeconstructionRightParts(conv.Operand);
                        }
                    }
                    break;
            }

            if (expr.Type is TupleTypeSymbol tupleType)
            {
                // https://github.com/dotnet/roslyn/issues/33011: Should include conversion.UnderlyingConversions[i].
                // For instance, Boxing conversions (see Deconstruction_ImplicitBoxingConversion_02) and
                // ImplicitNullable conversions (see Deconstruction_ImplicitNullableConversion_02).
                VisitRvalue(expr);
                var fields = tupleType.TupleElements;
                return fields.SelectAsArray((f, e) => (BoundExpression)new BoundFieldAccess(e.Syntax, e, f, constantValueOpt: null), expr);
            }

            throw ExceptionUtilities.Unreachable;
        }

        public override BoundNode VisitIncrementOperator(BoundIncrementOperator node)
        {
            Debug.Assert(!IsConditionalState);

            var operandType = VisitRvalueWithState(node.Operand);
            var operandLvalue = LvalueResultType;
            bool setResult = false;

            if (this.State.Reachable)
            {
                // https://github.com/dotnet/roslyn/issues/29961 Update increment method based on operand type.
                MethodSymbol incrementOperator = (node.OperatorKind.IsUserDefined() && (object)node.MethodOpt != null && node.MethodOpt.ParameterCount == 1) ? node.MethodOpt : null;
                TypeWithAnnotations targetTypeOfOperandConversion;
                AssignmentKind assignmentKind = AssignmentKind.Assignment;
                ParameterSymbol target = null;

                // https://github.com/dotnet/roslyn/issues/29961 Update conversion method based on operand type.
                if (node.OperandConversion.IsUserDefined && (object)node.OperandConversion.Method != null && node.OperandConversion.Method.ParameterCount == 1)
                {
                    targetTypeOfOperandConversion = node.OperandConversion.Method.ReturnTypeWithAnnotations;
                }
                else if ((object)incrementOperator != null)
                {
                    targetTypeOfOperandConversion = incrementOperator.Parameters[0].TypeWithAnnotations;
                    assignmentKind = AssignmentKind.Argument;
                    target = incrementOperator.Parameters[0];
                }
                else
                {
                    // Either a built-in increment, or an error case.
                    targetTypeOfOperandConversion = default;
                }

                TypeWithState resultOfOperandConversionType;

                if (targetTypeOfOperandConversion.HasType)
                {
                    // https://github.com/dotnet/roslyn/issues/29961 Should something special be done for targetTypeOfOperandConversion for lifted case?
                    resultOfOperandConversionType = ApplyConversion(
                        node.Operand,
                        node.Operand,
                        node.OperandConversion,
                        targetTypeOfOperandConversion,
                        operandType,
                        checkConversion: true,
                        fromExplicitCast: false,
                        useLegacyWarnings: false,
                        assignmentKind,
                        target,
                        reportTopLevelWarnings: true,
                        reportRemainingWarnings: true);
                }
                else
                {
                    resultOfOperandConversionType = operandType;
                }

                TypeWithState resultOfIncrementType;
                if (incrementOperator is null)
                {
                    resultOfIncrementType = resultOfOperandConversionType;
                }
                else
                {
                    resultOfIncrementType = incrementOperator.ReturnTypeWithAnnotations.ToTypeWithState();
                }

                var operandTypeWithAnnotations = operandType.ToTypeWithAnnotations();
                resultOfIncrementType = ApplyConversion(
                    node,
                    node,
                    node.ResultConversion,
                    operandTypeWithAnnotations,
                    resultOfIncrementType,
                    checkConversion: true,
                    fromExplicitCast: false,
                    useLegacyWarnings: false,
                    AssignmentKind.Assignment);

                // https://github.com/dotnet/roslyn/issues/29961 Check node.Type.IsErrorType() instead?
                if (!node.HasErrors)
                {
                    var op = node.OperatorKind.Operator();
                    ResultType = (op == UnaryOperatorKind.PrefixIncrement || op == UnaryOperatorKind.PrefixDecrement) ? resultOfIncrementType : operandType;
                    setResult = true;

                    TrackNullableStateForAssignment(node, targetType: operandLvalue, targetSlot: MakeSlot(node.Operand), valueType: resultOfIncrementType);
                }
            }

            if (!setResult)
            {
                SetNotNullResult(node);
            }

            return null;
        }

        public override BoundNode VisitCompoundAssignmentOperator(BoundCompoundAssignmentOperator node)
        {
            Visit(node.Left);
            TypeWithAnnotations leftLValueType = LvalueResultType;
            TypeWithState leftResultType = ResultType;

            Debug.Assert(!IsConditionalState);

            TypeWithState leftOnRightType = GetAdjustedResult(leftResultType, MakeSlot(node.Left));

            // https://github.com/dotnet/roslyn/issues/29962 Update operator based on inferred argument types.
            if ((object)node.Operator.LeftType != null)
            {
                // https://github.com/dotnet/roslyn/issues/29962 Ignoring top-level nullability of operator left parameter.
                leftOnRightType = ApplyConversion(
                    node.Left,
                    node.Left,
                    node.LeftConversion,
                    TypeWithAnnotations.Create(node.Operator.LeftType),
                    leftOnRightType,
                    checkConversion: true,
                    fromExplicitCast: false,
                    useLegacyWarnings: false,
                    AssignmentKind.Assignment,
                    reportTopLevelWarnings: false,
                    reportRemainingWarnings: true);
            }
            else
            {
                leftOnRightType = default;
            }

            TypeWithState resultType;
            TypeWithState rightType = VisitRvalueWithState(node.Right);
            if ((object)node.Operator.ReturnType != null)
            {
                if (node.Operator.Kind.IsUserDefined() && (object)node.Operator.Method != null && node.Operator.Method.ParameterCount == 2)
                {
                    ReportArgumentWarnings(node.Left, leftOnRightType, node.Operator.Method.Parameters[0]);
                    ReportArgumentWarnings(node.Right, rightType, node.Operator.Method.Parameters[1]);
                }

                resultType = InferResultNullability(node.Operator.Kind, node.Operator.Method, node.Operator.ReturnType, leftOnRightType, rightType);
                resultType = ApplyConversion(
                    node,
                    node,
                    node.FinalConversion,
                    leftLValueType,
                    resultType,
                    checkConversion: true,
                    fromExplicitCast: false,
                    useLegacyWarnings: false,
                    AssignmentKind.Assignment);
            }
            else
            {
                resultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
            }

            TrackNullableStateForAssignment(node, leftLValueType, MakeSlot(node.Left), resultType);
            ResultType = resultType;
            return null;
        }

        public override BoundNode VisitFixedLocalCollectionInitializer(BoundFixedLocalCollectionInitializer node)
        {
            var initializer = node.Expression;
            if (initializer.Kind == BoundKind.AddressOfOperator)
            {
                initializer = ((BoundAddressOfOperator)initializer).Operand;
            }

            VisitRvalue(initializer);
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitAddressOfOperator(BoundAddressOfOperator node)
        {
            SetNotNullResult(node);
            return null;
        }

        private void ReportArgumentWarnings(BoundExpression argument, TypeWithState argumentType, ParameterSymbol parameter)
        {
            var paramType = parameter.TypeWithAnnotations;
            ReportNullableAssignmentIfNecessary(argument, paramType, argumentType, useLegacyWarnings: false, AssignmentKind.Argument, target: parameter);

            if (!argumentType.HasNullType && IsNullabilityMismatch(paramType.Type, argumentType.Type))
            {
                ReportNullabilityMismatchInArgument(argument, argumentType.Type, parameter, paramType.Type, forOutput: false);
            }
        }

        private void ReportNullabilityMismatchInRefArgument(BoundExpression argument, TypeWithAnnotations argumentType, ParameterSymbol parameter, TypeWithAnnotations parameterType)
        {
            ReportSafetyDiagnostic(ErrorCode.WRN_NullabilityMismatchInArgument,
                argument.Syntax, argumentType, parameterType,
                new FormattedSymbol(parameter, SymbolDisplayFormat.ShortFormat),
                new FormattedSymbol(parameter.ContainingSymbol, SymbolDisplayFormat.MinimallyQualifiedFormat));
        }

        /// <summary>
        /// Report warning passing argument where nested nullability does not match
        /// parameter (e.g.: calling `void F(object[] o)` with `F(new[] { maybeNull })`).
        /// </summary>
        private void ReportNullabilityMismatchInArgument(BoundExpression argument, TypeSymbol argumentType, ParameterSymbol parameter, TypeSymbol parameterType, bool forOutput)
        {
            ReportSafetyDiagnostic(forOutput ? ErrorCode.WRN_NullabilityMismatchInArgumentForOutput : ErrorCode.WRN_NullabilityMismatchInArgument,
                argument.Syntax, argumentType, parameterType,
                new FormattedSymbol(parameter, SymbolDisplayFormat.ShortFormat),
                new FormattedSymbol(parameter.ContainingSymbol, SymbolDisplayFormat.MinimallyQualifiedFormat));
        }

        private TypeWithAnnotations GetDeclaredLocalResult(LocalSymbol local)
        {
            return _variableTypes.TryGetValue(local, out TypeWithAnnotations type) ?
                type :
                local.TypeWithAnnotations;
        }

        private TypeWithAnnotations GetDeclaredParameterResult(ParameterSymbol parameter)
        {
            return _variableTypes.TryGetValue(parameter, out TypeWithAnnotations type) ?
                type :
                parameter.TypeWithAnnotations;
        }

        public override BoundNode VisitBaseReference(BoundBaseReference node)
        {
            VisitThisOrBaseReference(node);
            return null;
        }

        public override BoundNode VisitFieldAccess(BoundFieldAccess node)
        {
            VisitMemberAccess(node, node.ReceiverOpt, node.FieldSymbol);
            return null;
        }

        public override BoundNode VisitPropertyAccess(BoundPropertyAccess node)
        {
            VisitMemberAccess(node, node.ReceiverOpt, node.PropertySymbol);
            return null;
        }

        public override BoundNode VisitIndexerAccess(BoundIndexerAccess node)
        {
            VisitIndexerAccess(node, out _);
            return null;
        }

        private void VisitIndexerAccess(BoundIndexerAccess node, out PropertySymbol indexer)
        {
            var receiverOpt = node.ReceiverOpt;
            VisitRvalue(receiverOpt);
            // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
            // after indices have been visited, and only if the receiver has not changed.
            CheckPossibleNullReceiver(receiverOpt);

            // https://github.com/dotnet/roslyn/issues/29964 Update indexer based on inferred receiver type.
            indexer = node.Indexer;

            VisitArguments(node, node.Arguments, node.ArgumentRefKindsOpt, node.Indexer, node.ArgsToParamsOpt, node.Expanded);

            LvalueResultType = node.Indexer.TypeWithAnnotations;
        }

        public override BoundNode VisitEventAccess(BoundEventAccess node)
        {
            VisitMemberAccess(node, node.ReceiverOpt, node.EventSymbol);
            return null;
        }

        private void VisitMemberAccess(BoundExpression node, BoundExpression receiverOpt, Symbol member)
        {
            Debug.Assert(!IsConditionalState);

            var receiverType = (receiverOpt != null) ? VisitRvalueWithState(receiverOpt) : default;

            SpecialMember? nullableOfTMember = null;
            if (!member.IsStatic)
            {
                member = AsMemberOfType(receiverType.Type, member);
                nullableOfTMember = GetNullableOfTMember(member);
                // https://github.com/dotnet/roslyn/issues/30598: For l-values, mark receiver as not null
                // after RHS has been visited, and only if the receiver has not changed.
                bool skipReceiverNullCheck = nullableOfTMember != SpecialMember.System_Nullable_T_get_Value;
                CheckPossibleNullReceiver(receiverOpt, checkNullableValueType: !skipReceiverNullCheck);
            }

            var type = member.GetTypeOrReturnType();
            var resultType = type.ToTypeWithState();

            // We are supposed to track information for the node. Use whatever we managed to
            // accumulate so far.
            if (PossiblyNullableType(resultType.Type))
            {
                int slot = MakeMemberSlot(receiverOpt, member);
                if (slot > 0 && slot < this.State.Capacity)
                {
                    var state = this.State[slot];
                    resultType = new TypeWithState(resultType.Type, state);
                }
            }

            Debug.Assert(!IsConditionalState);
            if (nullableOfTMember == SpecialMember.System_Nullable_T_get_HasValue && !(receiverOpt is null))
            {
                int containingSlot = MakeSlot(receiverOpt);
                if (containingSlot > 0)
                {
                    Split();
                    this.StateWhenTrue[containingSlot] = NullableFlowState.NotNull;
                }
            }

            SetResult(resultType, type);
        }

        private SpecialMember? GetNullableOfTMember(Symbol member)
        {
            if (member.Kind == SymbolKind.Property)
            {
                var getMethod = ((PropertySymbol)member.OriginalDefinition).GetMethod;
                if ((object)getMethod != null && getMethod.ContainingType.SpecialType == SpecialType.System_Nullable_T)
                {
                    if (getMethod == compilation.GetSpecialTypeMember(SpecialMember.System_Nullable_T_get_Value))
                    {
                        return SpecialMember.System_Nullable_T_get_Value;
                    }
                    if (getMethod == compilation.GetSpecialTypeMember(SpecialMember.System_Nullable_T_get_HasValue))
                    {
                        return SpecialMember.System_Nullable_T_get_HasValue;
                    }
                }
            }
            return null;
        }

        private int GetNullableOfTValueSlot(TypeSymbol containingType, int containingSlot, out Symbol valueProperty)
        {
            Debug.Assert(containingType.IsNullableType());
            Debug.Assert(TypeSymbol.Equals(GetSlotType(containingSlot), containingType, TypeCompareKind.ConsiderEverything2));

            var getValue = (MethodSymbol)compilation.GetSpecialTypeMember(SpecialMember.System_Nullable_T_get_Value);
            valueProperty = getValue?.AsMember((NamedTypeSymbol)containingType)?.AssociatedSymbol;
            return (valueProperty is null) ? -1 : GetOrCreateSlot(valueProperty, containingSlot);
        }

        protected override void VisitForEachExpression(BoundForEachStatement node)
        {
            var expr = node.Expression;
            VisitRvalue(expr);
            CheckPossibleNullReceiver(expr);
        }

        public override void VisitForEachIterationVariables(BoundForEachStatement node)
        {
            // declare and assign all iteration variables
            TypeWithAnnotations sourceType = node.EnumeratorInfoOpt?.ElementTypeWithAnnotations ?? default;
            TypeWithState sourceState = sourceType.ToTypeWithState();
            foreach (var iterationVariable in node.IterationVariables)
            {
                var state = NullableFlowState.NotNull;
                if (!sourceState.HasNullType)
                {
                    TypeWithAnnotations destinationType = iterationVariable.TypeWithAnnotations;

                    if (iterationVariable.IsRef)
                    {
                        // foreach (ref DestinationType variable in collection)
                        if (IsNullabilityMismatch(sourceType, destinationType))
                        {
                            var foreachSyntax = (ForEachStatementSyntax)node.Syntax;
                            ReportNullabilityMismatchInAssignment(foreachSyntax.Type, sourceType, destinationType);
                        }
                        state = sourceState.State;
                    }
                    else
                    {
                        // foreach (DestinationType variable in collection)
                        // foreach (var variable in collection)
                        // foreach (var (..., ...) in collection)
                        // and asynchronous variants
                        HashSet<DiagnosticInfo> useSiteDiagnostics = null;
                        Conversion conversion = _conversions.ClassifyImplicitConversionFromType(sourceType.Type, destinationType.Type, ref useSiteDiagnostics);
                        TypeWithState result = ApplyConversion(
                            node.IterationVariableType,
                            operandOpt: null,
                            conversion,
                            destinationType,
                            sourceState,
                            checkConversion: false,
                            fromExplicitCast: false,
                            useLegacyWarnings: false,
                            AssignmentKind.Assignment,
                            reportTopLevelWarnings: false,
                            reportRemainingWarnings: false);
                        if (destinationType.Type.IsReferenceType && destinationType.NullableAnnotation.IsNotAnnotated() && result.MayBeNull)
                        {
                            ReportNonSafetyDiagnostic(node.IterationVariableType.Syntax);
                        }

                        state = result.State;
                    }
                }

                int slot = GetOrCreateSlot(iterationVariable);
                if (slot > 0)
                {
                    this.State[slot] = state;
                }
            }
        }

        public override BoundNode VisitFromEndIndexExpression(BoundFromEndIndexExpression node)
        {
            var result = base.VisitFromEndIndexExpression(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitObjectInitializerMember(BoundObjectInitializerMember node)
        {
            // Should be handled by VisitObjectCreationExpression.
            throw ExceptionUtilities.Unreachable;
        }

        public override BoundNode VisitDynamicObjectInitializerMember(BoundDynamicObjectInitializerMember node)
        {
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitBadExpression(BoundBadExpression node)
        {
            var result = base.VisitBadExpression(node);
            var type = TypeWithAnnotations.Create(node.Type);
            LvalueResultType = type;
            return result;
        }

        public override BoundNode VisitTypeExpression(BoundTypeExpression node)
        {
            var result = base.VisitTypeExpression(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitTypeOrValueExpression(BoundTypeOrValueExpression node)
        {
            // These should not appear after initial binding except in error cases.
            var result = base.VisitTypeOrValueExpression(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitUnaryOperator(BoundUnaryOperator node)
        {
            Debug.Assert(!IsConditionalState);

            _ = base.VisitUnaryOperator(node);
            var argumentResult = ResultType;
            TypeWithState resultType;

            if (node.OperatorKind.IsUserDefined() && (object)node.MethodOpt != null && node.MethodOpt.ParameterCount == 1)
            {
                // Update method based on inferred operand type: see https://github.com/dotnet/roslyn/issues/29605.
                ReportArgumentWarnings(node.Operand, argumentResult, node.MethodOpt.Parameters[0]);
                if (node.OperatorKind.IsLifted())
                {
                    resultType = LiftedReturnType(node.MethodOpt.ReturnTypeWithAnnotations, argumentResult.State);
                }
                else
                {
                    resultType = node.MethodOpt.ReturnTypeWithAnnotations.ToTypeWithState();
                }
            }
            else
            {
                resultType = new TypeWithState(node.Type, node.OperatorKind.IsLifted() ? argumentResult.State : NullableFlowState.NotNull);
            }

            ResultType = resultType;
            return null;
        }

        public override BoundNode VisitPointerIndirectionOperator(BoundPointerIndirectionOperator node)
        {
            var result = base.VisitPointerIndirectionOperator(node);
            var type = TypeWithAnnotations.Create(node.Type);
            LvalueResultType = type;
            return result;
        }

        public override BoundNode VisitPointerElementAccess(BoundPointerElementAccess node)
        {
            var result = base.VisitPointerElementAccess(node);
            var type = TypeWithAnnotations.Create(node.Type);
            LvalueResultType = type;
            return result;
        }

        public override BoundNode VisitRefTypeOperator(BoundRefTypeOperator node)
        {
            VisitRvalue(node.Operand);
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitMakeRefOperator(BoundMakeRefOperator node)
        {
            var result = base.VisitMakeRefOperator(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitRefValueOperator(BoundRefValueOperator node)
        {
            var result = base.VisitRefValueOperator(node);
            var type = TypeWithAnnotations.Create(node.Type, node.NullableAnnotation);
            LvalueResultType = type;
            return result;
        }

        private TypeWithState InferResultNullability(BoundUserDefinedConditionalLogicalOperator node)
        {
            if (node.OperatorKind.IsLifted())
            {
                // https://github.com/dotnet/roslyn/issues/33879 Conversions: Lifted operator
                // Should this use the updated flow type and state?  How should it compute nullability?
                return new TypeWithState(node.Type, NullableFlowState.NotNull);
            }

            // Update method based on inferred operand types: see https://github.com/dotnet/roslyn/issues/29605.
            if ((object)node.LogicalOperator != null && node.LogicalOperator.ParameterCount == 2)
            {
                return node.LogicalOperator.ReturnTypeWithAnnotations.ToTypeWithState();
            }
            else
            {
                return default;
            }
        }

        protected override void AfterLeftChildOfBinaryLogicalOperatorHasBeenVisited(BoundExpression node, BoundExpression right, bool isAnd, bool isBool, ref LocalState leftTrue, ref LocalState leftFalse)
        {
            Debug.Assert(!IsConditionalState);
            TypeWithState leftType = ResultType;
            // https://github.com/dotnet/roslyn/issues/29605 Update operator methods based on inferred operand types.
            MethodSymbol logicalOperator = null;
            MethodSymbol trueFalseOperator = null;
            BoundExpression left = null;

            switch (node.Kind)
            {
                case BoundKind.BinaryOperator:
                    Debug.Assert(!((BoundBinaryOperator)node).OperatorKind.IsUserDefined());
                    break;
                case BoundKind.UserDefinedConditionalLogicalOperator:
                    var binary = (BoundUserDefinedConditionalLogicalOperator)node;
                    if (binary.LogicalOperator != null && binary.LogicalOperator.ParameterCount == 2)
                    {
                        logicalOperator = binary.LogicalOperator;
                        left = binary.Left;
                        trueFalseOperator = isAnd ? binary.FalseOperator : binary.TrueOperator;

                        if ((object)trueFalseOperator != null && trueFalseOperator.ParameterCount != 1)
                        {
                            trueFalseOperator = null;
                        }
                    }
                    break;
                default:
                    throw ExceptionUtilities.UnexpectedValue(node.Kind);
            }

            Debug.Assert(trueFalseOperator is null || ((object)logicalOperator != null && left != null));

            if ((object)trueFalseOperator != null)
            {
                ReportArgumentWarnings(left, leftType, trueFalseOperator.Parameters[0]);
            }

            if ((object)logicalOperator != null)
            {
                ReportArgumentWarnings(left, leftType, logicalOperator.Parameters[0]);
            }

            Visit(right);
            TypeWithState rightType = ResultType;

            ResultType = InferResultNullabilityOfBinaryLogicalOperator(node, leftType, rightType);

            if ((object)logicalOperator != null)
            {
                ReportArgumentWarnings(right, rightType, logicalOperator.Parameters[1]);
            }

            AfterRightChildOfBinaryLogicalOperatorHasBeenVisited(node, right, isAnd, isBool, ref leftTrue, ref leftFalse);
        }

        private TypeWithState InferResultNullabilityOfBinaryLogicalOperator(BoundExpression node, TypeWithState leftType, TypeWithState rightType)
        {
            switch (node.Kind)
            {
                case BoundKind.BinaryOperator:
                    return InferResultNullability((BoundBinaryOperator)node, leftType, rightType);
                case BoundKind.UserDefinedConditionalLogicalOperator:
                    return InferResultNullability((BoundUserDefinedConditionalLogicalOperator)node);
                default:
                    throw ExceptionUtilities.UnexpectedValue(node.Kind);
            }
        }

        public override BoundNode VisitAwaitExpression(BoundAwaitExpression node)
        {
            var result = base.VisitAwaitExpression(node);
            CheckPossibleNullReceiver(node.Expression);
            if (node.Type.IsValueType || node.HasErrors || node.AwaitableInfo.GetResult is null)
            {
                SetNotNullResult(node);
            }
            else
            {
                // Update method based on inferred receiver type: see https://github.com/dotnet/roslyn/issues/29605.
                ResultType = node.AwaitableInfo.GetResult.ReturnTypeWithAnnotations.ToTypeWithState();
            }

            return result;
        }

        public override BoundNode VisitTypeOfOperator(BoundTypeOfOperator node)
        {
            var result = base.VisitTypeOfOperator(node);
            ResultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
            return result;
        }

        public override BoundNode VisitMethodInfo(BoundMethodInfo node)
        {
            var result = base.VisitMethodInfo(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitFieldInfo(BoundFieldInfo node)
        {
            var result = base.VisitFieldInfo(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitDefaultExpression(BoundDefaultExpression node)
        {
            Debug.Assert(!this.IsConditionalState);

            var result = base.VisitDefaultExpression(node);
            TypeSymbol type = node.Type;
            if (EmptyStructTypeCache.IsTrackableStructType(type))
            {
                int slot = GetOrCreateObjectCreationPlaceholderSlot(node);
                if (slot > 0)
                {
                    this.State[slot] = NullableFlowState.NotNull;
                    InheritNullableStateOfTrackableStruct(type, slot, valueSlot: -1, isDefaultValue: true);
                }
            }

            // https://github.com/dotnet/roslyn/issues/33344: this fails to produce an updated tuple type for a default expression
            // (should produce nullable element types for those elements that are of reference types)
            ResultType = TypeWithState.ForType(type);

            if (ResultType.State == NullableFlowState.MaybeNull && RequiresSafetyWarningWhenNullIntroduced(ResultType.Type) && !node.IsSuppressed)
            {
                // For type parameters that cannot be annotated, the analysis must report those
                // places where null values first sneak in, like `default`, `null`, and `GetFirstOrDefault`.
                // This is one of those places.
                ReportSafetyDiagnostic(ErrorCode.WRN_DefaultExpressionMayIntroduceNullT, node.Syntax, GetTypeAsDiagnosticArgument(ResultType.Type));
            }

            return result;
        }

        public override BoundNode VisitIsOperator(BoundIsOperator node)
        {
            Debug.Assert(!this.IsConditionalState);

            var operand = node.Operand;
            var result = base.VisitIsOperator(node);
            Debug.Assert(node.Type.SpecialType == SpecialType.System_Boolean);

            if (operand.Type?.IsValueType == false)
            {
                var slotBuilder = ArrayBuilder<int>.GetInstance();
                GetSlotsToMarkAsNotNullable(operand, slotBuilder);
                if (slotBuilder.Count > 0)
                {
                    Split();
                    MarkSlotsAsNotNull(slotBuilder, ref StateWhenTrue);
                }
                slotBuilder.Free();
            }

            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitAsOperator(BoundAsOperator node)
        {
            var argumentType = VisitRvalueWithState(node.Operand);
            NullableFlowState resultState = NullableFlowState.NotNull;
            var type = node.Type;

            if (type.CanContainNull())
            {
                switch (node.Conversion.Kind)
                {
                    case ConversionKind.Identity:
                    case ConversionKind.ImplicitReference:
                    case ConversionKind.Boxing:
                    case ConversionKind.ImplicitNullable:
                        resultState = argumentType.State;
                        break;

                    default:
                        resultState = NullableFlowState.MaybeNull;
                        if (RequiresSafetyWarningWhenNullIntroduced(type))
                        {
                            ReportSafetyDiagnostic(ErrorCode.WRN_AsOperatorMayReturnNull, node.Syntax, type);
                        }
                        break;
                }
            }

            ResultType = new TypeWithState(type, resultState);
            return null;
        }

        public override BoundNode VisitSizeOfOperator(BoundSizeOfOperator node)
        {
            var result = base.VisitSizeOfOperator(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitArgList(BoundArgList node)
        {
            var result = base.VisitArgList(node);
            Debug.Assert(node.Type.SpecialType == SpecialType.System_RuntimeArgumentHandle);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitArgListOperator(BoundArgListOperator node)
        {
            VisitArgumentsEvaluate(node.Arguments, node.ArgumentRefKindsOpt);
            Debug.Assert(node.Type is null);
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitLiteral(BoundLiteral node)
        {
            var result = base.VisitLiteral(node);

            Debug.Assert(!IsConditionalState);
            ResultType = new TypeWithState(node.Type, node.Type?.CanContainNull() != false && node.ConstantValue?.IsNull == true ? NullableFlowState.MaybeNull : NullableFlowState.NotNull);

            return result;
        }

        public override BoundNode VisitPreviousSubmissionReference(BoundPreviousSubmissionReference node)
        {
            var result = base.VisitPreviousSubmissionReference(node);
            Debug.Assert(node.WasCompilerGenerated);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitHostObjectMemberReference(BoundHostObjectMemberReference node)
        {
            var result = base.VisitHostObjectMemberReference(node);
            Debug.Assert(node.WasCompilerGenerated);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitPseudoVariable(BoundPseudoVariable node)
        {
            var result = base.VisitPseudoVariable(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitRangeExpression(BoundRangeExpression node)
        {
            var result = base.VisitRangeExpression(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitRangeVariable(BoundRangeVariable node)
        {
            var result = base.VisitRangeVariable(node);
            SetNotNullResult(node); // https://github.com/dotnet/roslyn/issues/29863 Need to review this
            return result;
        }

        public override BoundNode VisitLabel(BoundLabel node)
        {
            var result = base.VisitLabel(node);
            SetUnknownResultNullability();
            return result;
        }

        public override BoundNode VisitDynamicMemberAccess(BoundDynamicMemberAccess node)
        {
            var receiver = node.Receiver;
            VisitRvalue(receiver);
            CheckPossibleNullReceiver(receiver);

            Debug.Assert(node.Type.IsDynamic());
            var result = TypeWithAnnotations.Create(node.Type);
            LvalueResultType = result;
            return null;
        }

        public override BoundNode VisitDynamicInvocation(BoundDynamicInvocation node)
        {
            VisitRvalue(node.Expression);
            VisitArgumentsEvaluate(node.Arguments, node.ArgumentRefKindsOpt);
            Debug.Assert(node.Type.IsDynamic());
            Debug.Assert(node.Type.IsReferenceType);
            var result = TypeWithAnnotations.Create(node.Type, NullableAnnotation.Oblivious);
            LvalueResultType = result;
            return null;
        }

        public override BoundNode VisitEventAssignmentOperator(BoundEventAssignmentOperator node)
        {
            VisitRvalue(node.ReceiverOpt);
            Debug.Assert(!IsConditionalState);
            var receiverOpt = node.ReceiverOpt;
            var @event = node.Event;
            if (!@event.IsStatic)
            {
                @event = (EventSymbol)AsMemberOfType(ResultType.Type, @event);
                // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
                // after arguments have been visited, and only if the receiver has not changed.
                CheckPossibleNullReceiver(receiverOpt);
            }
            VisitRvalue(node.Argument);
            // https://github.com/dotnet/roslyn/issues/31018: Check for delegate mismatch.
            SetNotNullResult(node); // https://github.com/dotnet/roslyn/issues/29969 Review whether this is the correct result
            return null;
        }

        public override BoundNode VisitDynamicObjectCreationExpression(BoundDynamicObjectCreationExpression node)
        {
            Debug.Assert(!IsConditionalState);
            var arguments = node.Arguments;
            var argumentResults = VisitArgumentsEvaluate(arguments, node.ArgumentRefKindsOpt);
            VisitObjectOrDynamicObjectCreation(node, arguments, argumentResults, node.InitializerExpressionOpt);
            return null;
        }

        public override BoundNode VisitObjectInitializerExpression(BoundObjectInitializerExpression node)
        {
            // Only reachable from bad expression. Otherwise handled in VisitObjectCreationExpression().
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitCollectionInitializerExpression(BoundCollectionInitializerExpression node)
        {
            // Only reachable from bad expression. Otherwise handled in VisitObjectCreationExpression().
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitDynamicCollectionElementInitializer(BoundDynamicCollectionElementInitializer node)
        {
            // Only reachable from bad expression. Otherwise handled in VisitObjectCreationExpression().
            SetNotNullResult(node);
            return null;
        }

        public override BoundNode VisitImplicitReceiver(BoundImplicitReceiver node)
        {
            var result = base.VisitImplicitReceiver(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitAnonymousPropertyDeclaration(BoundAnonymousPropertyDeclaration node)
        {
            throw ExceptionUtilities.Unreachable;
        }

        public override BoundNode VisitNoPiaObjectCreationExpression(BoundNoPiaObjectCreationExpression node)
        {
            var result = base.VisitNoPiaObjectCreationExpression(node);
            ResultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
            return result;
        }

        public override BoundNode VisitNewT(BoundNewT node)
        {
            VisitObjectOrDynamicObjectCreation(node, ImmutableArray<BoundExpression>.Empty, ImmutableArray<VisitResult>.Empty, node.InitializerExpressionOpt);
            return null;
        }

        public override BoundNode VisitArrayInitialization(BoundArrayInitialization node)
        {
            var result = base.VisitArrayInitialization(node);
            SetNotNullResult(node);
            return result;
        }

        private void SetUnknownResultNullability()
        {
            ResultType = default;
        }

        public override BoundNode VisitStackAllocArrayCreation(BoundStackAllocArrayCreation node)
        {
            var result = base.VisitStackAllocArrayCreation(node);
            Debug.Assert(node.Type is null || node.Type.IsPointerType() || node.Type.IsRefLikeType);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitDynamicIndexerAccess(BoundDynamicIndexerAccess node)
        {
            var receiver = node.ReceiverOpt;
            VisitRvalue(receiver);
            // https://github.com/dotnet/roslyn/issues/30598: Mark receiver as not null
            // after indices have been visited, and only if the receiver has not changed.
            CheckPossibleNullReceiver(receiver);
            VisitArgumentsEvaluate(node.Arguments, node.ArgumentRefKindsOpt);
            Debug.Assert(node.Type.IsDynamic());
            var result = TypeWithAnnotations.Create(node.Type, NullableAnnotation.Oblivious);
            LvalueResultType = result;
            return null;
        }

        private void CheckPossibleNullReceiver(BoundExpression receiverOpt, bool checkNullableValueType = false)
        {
            Debug.Assert(!this.IsConditionalState);
            if (receiverOpt != null && this.State.Reachable)
            {
                var resultTypeSymbol = ResultType.Type;
                if (resultTypeSymbol is null)
                {
                    return;
                }
#if DEBUG
                Debug.Assert(receiverOpt.Type is null || AreCloseEnough(receiverOpt.Type, resultTypeSymbol));
#endif
                if (ResultType.MayBeNull)
                {
                    bool isValueType = resultTypeSymbol.IsValueType;
                    if (isValueType && (!checkNullableValueType || !resultTypeSymbol.IsNullableTypeOrTypeParameter() || resultTypeSymbol.GetNullableUnderlyingType().IsErrorType()))
                    {
                        return;
                    }

                    ReportSafetyDiagnostic(isValueType ? ErrorCode.WRN_NullableValueTypeMayBeNull : ErrorCode.WRN_NullReferenceReceiver, receiverOpt.Syntax);
                }

                LearnFromNonNullTest(receiverOpt, ref this.State);
            }
        }

        private static bool IsNullabilityMismatch(TypeWithAnnotations type1, TypeWithAnnotations type2)
        {
            // Note, when we are paying attention to nullability, we ignore insignificant differences and oblivious mismatch.
            // See TypeCompareKind.UnknownNullableModifierMatchesAny and TypeCompareKind.IgnoreInsignificantNullableModifiersDifference
            return type1.Equals(type2, TypeCompareKind.AllIgnoreOptions) &&
                !type1.Equals(type2, TypeCompareKind.AllIgnoreOptions & ~TypeCompareKind.IgnoreNullableModifiersForReferenceTypes);
        }

        private static bool IsNullabilityMismatch(TypeSymbol type1, TypeSymbol type2)
        {
            // Note, when we are paying attention to nullability, we ignore insignificant differences and oblivious mismatch.
            // See TypeCompareKind.UnknownNullableModifierMatchesAny and TypeCompareKind.IgnoreInsignificantNullableModifiersDifference
            return type1.Equals(type2, TypeCompareKind.AllIgnoreOptions) &&
                !type1.Equals(type2, TypeCompareKind.AllIgnoreOptions & ~TypeCompareKind.IgnoreNullableModifiersForReferenceTypes);
        }

        public override BoundNode VisitQueryClause(BoundQueryClause node)
        {
            var result = base.VisitQueryClause(node);
            SetNotNullResult(node); // https://github.com/dotnet/roslyn/issues/29863 Implement nullability analysis in LINQ queries
            return result;
        }

        public override BoundNode VisitNameOfOperator(BoundNameOfOperator node)
        {
            var result = base.VisitNameOfOperator(node);
            ResultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
            return result;
        }

        public override BoundNode VisitNamespaceExpression(BoundNamespaceExpression node)
        {
            var result = base.VisitNamespaceExpression(node);
            SetUnknownResultNullability();
            return result;
        }

        public override BoundNode VisitInterpolatedString(BoundInterpolatedString node)
        {
            var result = base.VisitInterpolatedString(node);
            ResultType = new TypeWithState(node.Type, NullableFlowState.NotNull);
            return result;
        }

        public override BoundNode VisitStringInsert(BoundStringInsert node)
        {
            var result = base.VisitStringInsert(node);
            SetUnknownResultNullability();
            return result;
        }

        public override BoundNode VisitConvertedStackAllocExpression(BoundConvertedStackAllocExpression node)
        {
            var result = base.VisitConvertedStackAllocExpression(node);
            SetNotNullResult(node);
            return result;
        }

        public override BoundNode VisitDiscardExpression(BoundDiscardExpression node)
        {
            var result = TypeWithAnnotations.Create(node.Type);
            var rValueType = new TypeWithState(node.Type, NullableFlowState.MaybeNull);
            SetResult(rValueType, result);
            return null;
        }

        public override BoundNode VisitThrowExpression(BoundThrowExpression node)
        {
            VisitThrow(node.Expression);
            ResultType = default;
            return null;
        }

        public override BoundNode VisitThrowStatement(BoundThrowStatement node)
        {
            VisitThrow(node.ExpressionOpt);
            return null;
        }

        private void VisitThrow(BoundExpression expr)
        {
            if (expr != null)
            {
                var result = VisitRvalueWithState(expr);
                // Cases:
                // null
                // null!
                // Other (typed) expression, including suppressed ones
                if (result.MayBeNull)
                {
                    ReportSafetyDiagnostic(ErrorCode.WRN_PossibleNull, expr.Syntax);
                }
            }
            SetUnreachable();
        }

        public override BoundNode VisitYieldReturnStatement(BoundYieldReturnStatement node)
        {
            BoundExpression expr = node.Expression;
            if (expr == null)
            {
                return null;
            }
            var method = (MethodSymbol)_symbol;
            TypeWithAnnotations elementType = InMethodBinder.GetIteratorElementTypeFromReturnType(compilation, RefKind.None,
                method.ReturnType, errorLocationNode: null, diagnostics: null).elementType;

            _ = VisitOptionalImplicitConversion(expr, elementType, useLegacyWarnings: false, AssignmentKind.Return);
            return null;
        }

        protected override void VisitCatchBlock(BoundCatchBlock node, ref LocalState finallyState)
        {
            if (node.Locals.Length > 0)
            {
                LocalSymbol local = node.Locals[0];
                if (local.DeclarationKind == LocalDeclarationKind.CatchVariable)
                {
                    int slot = GetOrCreateSlot(local);
                    if (slot > 0)
                        this.State[slot] = NullableFlowState.NotNull;
                }
            }

            base.VisitCatchBlock(node, ref finallyState);
        }

        public override BoundNode VisitLockStatement(BoundLockStatement node)
        {
            VisitRvalue(node.Argument);
            CheckPossibleNullReceiver(node.Argument);
            VisitStatement(node.Body);
            return null;
        }

        public override BoundNode VisitAttribute(BoundAttribute node)
        {
            VisitArguments(node, node.ConstructorArguments, ImmutableArray<RefKind>.Empty, node.Constructor, argsToParamsOpt: node.ConstructorArgumentsToParamsOpt, expanded: node.ConstructorExpanded);
            foreach (var assignment in node.NamedArguments)
            {
                Visit(assignment);
            }

            SetNotNullResult(node);
            return null;
        }

        protected override string Dump(LocalState state)
        {
            if (!state.Reachable)
                return "unreachable";

            var pooledBuilder = PooledStringBuilder.GetInstance();
            var builder = pooledBuilder.Builder;
            for (int i = 0; i < state.Capacity; i++)
            {
                if (nameForSlot(i) is string name)
                {
                    builder.Append(name);
                    builder.Append(state[i] == NullableFlowState.MaybeNull ? "?" : "!");
                }
            }

            return pooledBuilder.ToStringAndFree();

            string nameForSlot(int slot)
            {
                if (slot < 0)
                    return null;
                VariableIdentifier id = this.variableBySlot[slot];
                var name = id.Symbol?.Name;
                if (name == null)
                    return null;
                return nameForSlot(id.ContainingSlot) is string containingSlotName
                    ? containingSlotName + "." + name : name;
            }
        }

        protected override void Meet(ref LocalState self, ref LocalState other)
        {
            if (!self.Reachable)
                return;

            if (!other.Reachable)
            {
                self = other.Clone();
                return;
            }

            if (self.Capacity != other.Capacity)
            {
                Normalize(ref self);
                Normalize(ref other);
            }

            for (int slot = 1; slot < self.Capacity; slot++)
            {
                NullableFlowState selfState = self[slot];
                NullableFlowState otherState = other[slot];
                NullableFlowState union = selfState.Meet(otherState);

                if (selfState != union)
                {
                    self[slot] = union;
                }
            }
        }

        protected override bool Join(ref LocalState self, ref LocalState other)
        {
            if (!other.Reachable)
                return false;

            if (!self.Reachable)
            {
                self = other.Clone();
                return true;
            }

            bool result = false;

            if (self.Capacity != other.Capacity)
            {
                Normalize(ref self);
                Normalize(ref other);
            }

            for (int slot = 1; slot < self.Capacity; slot++)
            {
                NullableFlowState selfAnnotation = self[slot];
                NullableFlowState intersection = selfAnnotation.Join(other[slot]);
                if (selfAnnotation != intersection)
                {
                    self[slot] = intersection;
                    result = true;
                }
            }

            return result;
        }

        [DebuggerDisplay("{GetDebuggerDisplay(), nq}")]
#if REFERENCE_STATE
        internal class LocalState : ILocalState
#else
        internal struct LocalState : ILocalState
#endif
        {
            private ArrayBuilder<NullableFlowState> _state;
            public bool Reachable { get; }

            internal LocalState(bool reachable, ArrayBuilder<NullableFlowState> state)
            {
                this.Reachable = reachable;
                this._state = state;
            }

            internal int Capacity => _state?.Count ?? 0;

            internal void EnsureCapacity(int capacity)
            {
                if (!Reachable)
                {
                    return;
                }

                if (_state == null)
                {
                    _state = new ArrayBuilder<NullableFlowState>(capacity);
                }

                if (_state.Count < capacity)
                {
                    _state.Count = capacity;
                }
            }

            internal NullableFlowState this[int slot]
            {
                get
                {
                    if (slot < Capacity && this.Reachable)
                    {
                        return _state[slot];
                    }

                    return NullableFlowState.NotNull;
                }
                set
                {
                    if (this.Reachable)
                    {
                        // All variables are be considered not null in unreachable code.
                        // Moreover, no states should be modified in unreachable code, as there is only one unreachable state.
                        EnsureCapacity(slot + 1);
                        _state[slot] = value;
                    }
                }
            }

            /// <summary>
            /// Produce a duplicate of this flow analysis state.
            /// </summary>
            /// <returns></returns>
            public LocalState Clone()
            {
                ArrayBuilder<NullableFlowState> clone;

                if (_state == null)
                {
                    clone = null;
                }
                else
                {
                    clone = new ArrayBuilder<NullableFlowState>(_state.Count);
                    clone.Count = 0;
                    clone.AddRange(_state);
                }

                return new LocalState(Reachable, clone);
            }

            internal string GetDebuggerDisplay()
            {
                var pooledBuilder = PooledStringBuilder.GetInstance();
                var builder = pooledBuilder.Builder;
                builder.Append(" ");
                for (int i = this.Capacity - 1; i >= 0; i--)
                {
                    string append;

                    switch (_state[i])
                    {
                        case NullableFlowState.NotNull:
                            append = "!";
                            break;
                        case NullableFlowState.MaybeNull:
                            append = "?";
                            break;
                        default:
                            throw ExceptionUtilities.UnexpectedValue(_state[i]);
                    }

                    builder.Append(append);
                }

                return pooledBuilder.ToStringAndFree();
            }
        }
    }
}