Enable nullable reference types for symbol extensions

src/Workspaces/CSharp/Portable/Extensions/ConversionExtensions.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.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
 using Microsoft.CodeAnalysis.CSharp.Syntax;

src/Workspaces/CSharp/Portable/Extensions/ILocalSymbolExtensions.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.
 
+#nullable enable
+
 using System.Linq;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 

src/Workspaces/CSharp/Portable/Extensions/ITypeParameterSymbolExtensions.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.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;

src/Workspaces/CSharp/Portable/Extensions/ITypeSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -97,7 +99,7 @@ public static bool ContainingTypesOrSelfHasUnsafeKeyword(this ITypeSymbol contai
             return false;
         }
 
-        public static async Task<ISymbol> FindApplicableAlias(this ITypeSymbol type, int position, SemanticModel semanticModel, CancellationToken cancellationToken)
+        public static async Task<ISymbol?> FindApplicableAlias(this ITypeSymbol type, int position, SemanticModel semanticModel, CancellationToken cancellationToken)
         {
             try
             {
@@ -109,7 +111,7 @@ public static async Task<ISymbol> FindApplicableAlias(this ITypeSymbol type, int
 
                 var root = await semanticModel.SyntaxTree.GetRootAsync(cancellationToken).ConfigureAwait(false);
 
-                var applicableUsings = GetApplicableUsings(position, root as CompilationUnitSyntax);
+                var applicableUsings = GetApplicableUsings(position, (CompilationUnitSyntax)root);
                 foreach (var applicableUsing in applicableUsings)
                 {
                     var alias = semanticModel.GetOriginalSemanticModel().GetDeclaredSymbol(applicableUsing, cancellationToken);

src/Workspaces/CSharp/Portable/Extensions/SymbolDisplayPartExtensions.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.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis;
 using Microsoft.CodeAnalysis.CSharp;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
@@ -10,7 +12,7 @@ namespace Microsoft.CodeAnalysis.CSharp.Extensions
 {
     internal static class SymbolDisplayPartExtensions
     {
-        public static SymbolDisplayPart MassageErrorTypeNames(this SymbolDisplayPart part, string replacement = null)
+        public static SymbolDisplayPart MassageErrorTypeNames(this SymbolDisplayPart part, string? replacement = null)
         {
             if (part.Kind == SymbolDisplayPartKind.ErrorTypeName)
             {

src/Workspaces/Core/Portable/Shared/Extensions/IMethodSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis;
@@ -367,7 +370,7 @@ public static PredefinedOperator GetPredefinedOperator(this IMethodSymbol symbol
         /// the first parameter is of <see cref="object"/> type and the second
         /// parameter inherits from or equals <see cref="EventArgs"/> type.
         /// </summary>
-        public static bool HasEventHandlerSignature(this IMethodSymbol method, INamedTypeSymbol eventArgsType)
+        public static bool HasEventHandlerSignature(this IMethodSymbol method, [NotNullWhen(returnValue: true)] INamedTypeSymbol? eventArgsType)
             => eventArgsType != null &&
                method.Parameters.Length == 2 &&
                method.Parameters[0].Type.SpecialType == SpecialType.System_Object &&

src/Workspaces/Core/Portable/Shared/Extensions/INamedTypeSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis;
@@ -13,7 +16,7 @@ namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static partial class INamedTypeSymbolExtensions
     {
-        public static IEnumerable<INamedTypeSymbol> GetBaseTypesAndThis(this INamedTypeSymbol namedType)
+        public static IEnumerable<INamedTypeSymbol> GetBaseTypesAndThis(this INamedTypeSymbol? namedType)
         {
             var current = namedType;
             while (current != null)
@@ -23,19 +26,19 @@ public static IEnumerable<INamedTypeSymbol> GetBaseTypesAndThis(this INamedTypeS
             }
         }
 
-        public static IEnumerable<ITypeParameterSymbol> GetAllTypeParameters(this INamedTypeSymbol symbol)
+        public static IEnumerable<ITypeParameterSymbol> GetAllTypeParameters(this INamedTypeSymbol? symbol)
         {
             var stack = GetContainmentStack(symbol);
             return stack.SelectMany(n => n.TypeParameters);
         }
 
-        public static IEnumerable<ITypeSymbol> GetAllTypeArguments(this INamedTypeSymbol symbol)
+        public static IEnumerable<ITypeSymbol> GetAllTypeArguments(this INamedTypeSymbol? symbol)
         {
             var stack = GetContainmentStack(symbol);
             return stack.SelectMany(n => n.TypeArguments);
         }
 
-        private static Stack<INamedTypeSymbol> GetContainmentStack(INamedTypeSymbol symbol)
+        private static Stack<INamedTypeSymbol> GetContainmentStack(INamedTypeSymbol? symbol)
         {
             var stack = new Stack<INamedTypeSymbol>();
             for (var current = symbol; current != null; current = current.ContainingType)
@@ -46,7 +49,7 @@ private static Stack<INamedTypeSymbol> GetContainmentStack(INamedTypeSymbol symb
             return stack;
         }
 
-        public static bool IsContainedWithin(this INamedTypeSymbol symbol, INamedTypeSymbol outer)
+        public static bool IsContainedWithin([NotNullWhen(returnValue: true)] this INamedTypeSymbol? symbol, INamedTypeSymbol outer)
         {
             // TODO(cyrusn): Should we be using OriginalSymbol here?
             for (var current = symbol; current != null; current = current.ContainingType)
@@ -60,7 +63,7 @@ public static bool IsContainedWithin(this INamedTypeSymbol symbol, INamedTypeSym
             return false;
         }
 
-        public static ISymbol FindImplementationForAbstractMember(this INamedTypeSymbol type, ISymbol symbol)
+        public static ISymbol? FindImplementationForAbstractMember(this INamedTypeSymbol? type, ISymbol symbol)
         {
             if (symbol.IsAbstract)
             {
@@ -71,7 +74,7 @@ public static ISymbol FindImplementationForAbstractMember(this INamedTypeSymbol
             return null;
         }
 
-        internal static ISymbol GetOverriddenMember(this ISymbol symbol)
+        internal static ISymbol? GetOverriddenMember(this ISymbol? symbol)
         {
             switch (symbol)
             {
@@ -129,7 +132,7 @@ private static bool IsInterfacePropertyImplemented(INamedTypeSymbol classOrStruc
 
             // local functions
 
-            static bool IsAccessorImplemented(IMethodSymbol accessor, INamedTypeSymbol classOrStructType)
+            static bool IsAccessorImplemented(IMethodSymbol? accessor, INamedTypeSymbol classOrStructType)
             {
                 return accessor == null || !IsImplementable(accessor) || classOrStructType.FindImplementationForInterfaceMember(accessor) != null;
             }
@@ -305,7 +308,7 @@ static bool IsPropertyWithInaccessibleImplementableAccessor(ISymbol member, ISym
                 return IsInaccessibleImplementableAccessor(property.GetMethod, within) || IsInaccessibleImplementableAccessor(property.SetMethod, within);
             }
 
-            static bool IsInaccessibleImplementableAccessor(IMethodSymbol accessor, ISymbol within)
+            static bool IsInaccessibleImplementableAccessor(IMethodSymbol? accessor, ISymbol within)
             {
                 return accessor != null && IsImplementable(accessor) && !accessor.IsAccessibleWithin(within);
             }
@@ -434,7 +437,7 @@ static bool IsInaccessibleImplementableAccessor(IMethodSymbol accessor, ISymbol
             }
         }
 
-        private static ISymbol IsAttributeNamedParameter(
+        private static ISymbol? IsAttributeNamedParameter(
             ISymbol symbol,
             ISymbol within)
         {

src/Workspaces/Core/Portable/Shared/Extensions/INamespaceOrTypeSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -29,7 +31,7 @@ public static string GetShortName(this INamespaceOrTypeSymbol symbol)
             return symbol.ToDisplayString(s_shortNameFormat);
         }
 
-        public static IEnumerable<IPropertySymbol> GetIndexers(this INamespaceOrTypeSymbol symbol)
+        public static IEnumerable<IPropertySymbol> GetIndexers(this INamespaceOrTypeSymbol? symbol)
         {
             return symbol == null
                 ? SpecializedCollections.EmptyEnumerable<IPropertySymbol>()
@@ -73,7 +75,7 @@ public static IReadOnlyList<string> GetNameParts(this INamespaceOrTypeSymbol sym
             return names1.Count - names2.Count;
         }
 
-        private static void GetNameParts(INamespaceOrTypeSymbol namespaceOrTypeSymbol, List<string> result)
+        private static void GetNameParts(INamespaceOrTypeSymbol? namespaceOrTypeSymbol, List<string> result)
         {
             if (namespaceOrTypeSymbol == null || (namespaceOrTypeSymbol.IsNamespace && ((INamespaceSymbol)namespaceOrTypeSymbol).IsGlobalNamespace))
             {

src/Workspaces/Core/Portable/Shared/Extensions/INamespaceSymbolExtensions.Comparer.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.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Linq;
 using Roslyn.Utilities;
@@ -8,14 +10,14 @@ namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal partial class INamespaceSymbolExtensions
     {
-        private class Comparer : IEqualityComparer<INamespaceSymbol>
+        private class Comparer : IEqualityComparer<INamespaceSymbol?>
         {
-            public bool Equals(INamespaceSymbol x, INamespaceSymbol y)
+            public bool Equals(INamespaceSymbol? x, INamespaceSymbol? y)
             {
                 return GetNameParts(x).SequenceEqual(GetNameParts(y));
             }
 
-            public int GetHashCode(INamespaceSymbol obj)
+            public int GetHashCode(INamespaceSymbol? obj)
             {
                 return GetNameParts(obj).Aggregate(0, (a, v) => Hash.Combine(v, a));
             }

src/Workspaces/Core/Portable/Shared/Extensions/INamespaceSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
@@ -19,14 +21,14 @@ internal static partial class INamespaceSymbolExtensions
         public static readonly Comparison<INamespaceSymbol> CompareNamespaces = CompareTo;
         public static readonly IEqualityComparer<INamespaceSymbol> EqualityComparer = new Comparer();
 
-        private static List<string> GetNameParts(INamespaceSymbol namespaceSymbol)
+        private static List<string> GetNameParts(INamespaceSymbol? namespaceSymbol)
         {
             var result = new List<string>();
             GetNameParts(namespaceSymbol, result);
             return result;
         }
 
-        private static void GetNameParts(INamespaceSymbol namespaceSymbol, List<string> result)
+        private static void GetNameParts(INamespaceSymbol? namespaceSymbol, List<string> result)
         {
             if (namespaceSymbol == null || namespaceSymbol.IsGlobalNamespace)
             {
@@ -143,11 +145,11 @@ public static int CompareTo(this INamespaceSymbol n1, INamespaceSymbol n2)
             }
         }
 
-        public static INamespaceSymbol GetQualifiedNamespace(
+        public static INamespaceSymbol? GetQualifiedNamespace(
             this INamespaceSymbol globalNamespace,
             string namespaceName)
         {
-            var namespaceSymbol = globalNamespace;
+            INamespaceSymbol? namespaceSymbol = globalNamespace;
             foreach (var name in namespaceName.Split('.'))
             {
                 var members = namespaceSymbol.GetMembers(name);
@@ -155,7 +157,7 @@ public static int CompareTo(this INamespaceSymbol n1, INamespaceSymbol n2)
                         ? members.First() as INamespaceSymbol
                         : null;
 
-                if ((object)namespaceSymbol == null)
+                if (namespaceSymbol is null)
                 {
                     break;
                 }

src/Workspaces/Core/Portable/Shared/Extensions/IPropertySymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -36,7 +38,7 @@ public static IPropertySymbol RenameParameters(this IPropertySymbol property, IL
         }
 
         public static IPropertySymbol RemoveAttributeFromParameters(
-            this IPropertySymbol property, INamedTypeSymbol[] attributesToRemove)
+            this IPropertySymbol property, INamedTypeSymbol?[]? attributesToRemove)
         {
             if (attributesToRemove == null)
             {
@@ -44,7 +46,7 @@ public static IPropertySymbol RenameParameters(this IPropertySymbol property, IL
             }
 
             bool shouldRemoveAttribute(AttributeData a) =>
-                attributesToRemove.Where(attr => attr != null).Any(attr => attr.Equals(a.AttributeClass));
+                attributesToRemove.Where(attr => attr != null).Any(attr => attr!.Equals(a.AttributeClass));
 
             var someParameterHasAttribute = property.Parameters
                 .Any(p => p.GetAttributes().Any(shouldRemoveAttribute));
@@ -75,7 +77,7 @@ public static IPropertySymbol RenameParameters(this IPropertySymbol property, IL
         public static bool IsWritableInConstructor(this IPropertySymbol property)
             => (property.SetMethod != null || ContainsBackingField(property));
 
-        public static IFieldSymbol GetBackingFieldIfAny(this IPropertySymbol property)
+        public static IFieldSymbol? GetBackingFieldIfAny(this IPropertySymbol property)
             => property.ContainingType.GetMembers()
                 .OfType<IFieldSymbol>()
                 .FirstOrDefault(f => property.Equals(f.AssociatedSymbol));

src/Workspaces/Core/Portable/Shared/Extensions/ITypeParameterSymbolExtensions.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.
 
+#nullable enable
+
 using System.Linq;
 using Roslyn.Utilities;
 
@@ -7,12 +9,12 @@ namespace Microsoft.CodeAnalysis.Shared.Extensions
 {
     internal static class ITypeParameterSymbolExtensions
     {
-        public static INamedTypeSymbol GetNamedTypeSymbolConstraint(this ITypeParameterSymbol typeParameter)
+        public static INamedTypeSymbol? GetNamedTypeSymbolConstraint(this ITypeParameterSymbol typeParameter)
         {
             return typeParameter.ConstraintTypes.Select(GetNamedTypeSymbol).WhereNotNull().FirstOrDefault();
         }
 
-        private static INamedTypeSymbol GetNamedTypeSymbol(ITypeSymbol type)
+        private static INamedTypeSymbol? GetNamedTypeSymbol(ITypeSymbol type)
         {
             return type is INamedTypeSymbol
                 ? (INamedTypeSymbol)type

src/Workspaces/Core/Portable/Shared/Extensions/ITypeSymbolExtensions.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.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -22,7 +25,7 @@ internal static partial class ITypeSymbolExtensions
         private const string DefaultParameterName = "p";
         private const string DefaultBuiltInParameterName = "v";
 
-        public static bool CanAddNullCheck(this ITypeSymbol type)
+        public static bool CanAddNullCheck([NotNullWhen(returnValue: true)] this ITypeSymbol? type)
             => type != null && (type.IsReferenceType || type.IsNullable());
 
         public static IList<INamedTypeSymbol> GetAllInterfacesIncludingThis(this ITypeSymbol type)
@@ -39,45 +42,46 @@ public static IList<INamedTypeSymbol> GetAllInterfacesIncludingThis(this ITypeSy
             return allInterfaces;
         }
 
-        public static bool IsAbstractClass(this ITypeSymbol symbol)
+        public static bool IsAbstractClass([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.TypeKind == TypeKind.Class && symbol.IsAbstract;
         }
 
-        public static bool IsSystemVoid(this ITypeSymbol symbol)
+        public static bool IsSystemVoid([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.SpecialType == SpecialType.System_Void;
         }
 
-        public static bool IsNullable(this ITypeSymbol symbol)
+        public static bool IsNullable([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
             => symbol?.OriginalDefinition.SpecialType == SpecialType.System_Nullable_T;
 
-        public static bool IsModuleType(this ITypeSymbol symbol)
+        public static bool IsModuleType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.TypeKind == TypeKind.Module;
         }
 
-        public static bool IsInterfaceType(this ITypeSymbol symbol)
+        public static bool IsInterfaceType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.TypeKind == TypeKind.Interface;
         }
 
-        public static bool IsDelegateType(this ITypeSymbol symbol)
+        public static bool IsDelegateType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.TypeKind == TypeKind.Delegate;
         }
 
-        public static bool IsStructType(this ITypeSymbol symbol)
+        public static bool IsStructType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.TypeKind == TypeKind.Struct;
         }
 
-        public static bool IsAnonymousType(this INamedTypeSymbol symbol)
+        public static bool IsAnonymousType([NotNullWhen(returnValue: true)] this INamedTypeSymbol? symbol)
         {
             return symbol?.IsAnonymousType == true;
         }
 
-        public static ITypeSymbol RemoveNullableIfPresent(this ITypeSymbol symbol)
+        [return: NotNullIfNotNull(parameterName: "symbol")]
+        public static ITypeSymbol? RemoveNullableIfPresent([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             if (symbol.IsNullable())
             {
@@ -216,14 +220,14 @@ public static ITypeSymbol RemoveNullableIfPresent(this ITypeSymbol symbol)
 
             // local functions
 
-            static Task<Compilation> GetCompilationOrNullAsync(Project project, CancellationToken cancellationToken)
+            static Task<Compilation> GetCompilationOrNullAsync(Project? project, CancellationToken cancellationToken)
                 => project?.GetCompilationAsync(cancellationToken) ?? SpecializedTasks.Default<Compilation>();
         }
 
 
         private static HashSet<INamedTypeSymbol> GetOriginalInterfacesAndTheirBaseInterfaces(
             this ITypeSymbol type,
-            HashSet<INamedTypeSymbol> symbols = null)
+            HashSet<INamedTypeSymbol>? symbols = null)
         {
             symbols ??= new HashSet<INamedTypeSymbol>(SymbolEquivalenceComparer.Instance);
 
@@ -236,7 +240,7 @@ static Task<Compilation> GetCompilationOrNullAsync(Project project, Cancellation
             return symbols;
         }
 
-        public static ISymbol FindImplementations(
+        public static ISymbol? FindImplementations(
             this ITypeSymbol typeSymbol,
             ISymbol constructedInterfaceMember,
             Workspace workspace)
@@ -265,7 +269,7 @@ where SymbolEquivalenceComparer.Instance.Equals(explicitInterfaceMethod, constru
                 select member;
 
             var provider = workspace.Services.GetLanguageServices(typeSymbol.Language);
-            var semanticFacts = provider.GetService<ISemanticFactsService>();
+            var semanticFacts = provider.GetRequiredService<ISemanticFactsService>();
 
             // Even if a language only supports explicit interface implementation, we
             // can't enforce it for types from metadata. For example, a VB symbol
@@ -279,7 +283,7 @@ where SymbolEquivalenceComparer.Instance.Equals(explicitInterfaceMethod, constru
                 return explicitMatches.FirstOrDefault();
             }
 
-            var syntaxFacts = provider.GetService<ISyntaxFactsService>();
+            var syntaxFacts = provider.GetRequiredService<ISyntaxFactsService>();
             var implicitMatches =
                 from baseType in typeSymbol.GetBaseTypesAndThis()
                 from member in baseType.GetMembers(constructedInterfaceMember.Name).OfType<TSymbol>()
@@ -291,7 +295,7 @@ where SymbolEquivalenceComparer.Instance.Equals(explicitInterfaceMethod, constru
             return explicitMatches.FirstOrDefault() ?? implicitMatches.FirstOrDefault();
         }
 
-        public static IEnumerable<ITypeSymbol> GetBaseTypesAndThis(this ITypeSymbol type)
+        public static IEnumerable<ITypeSymbol> GetBaseTypesAndThis(this ITypeSymbol? type)
         {
             var current = type;
             while (current != null)
@@ -311,7 +315,7 @@ public static IEnumerable<INamedTypeSymbol> GetBaseTypes(this ITypeSymbol type)
             }
         }
 
-        public static IEnumerable<ITypeSymbol> GetContainingTypesAndThis(this ITypeSymbol type)
+        public static IEnumerable<ITypeSymbol> GetContainingTypesAndThis(this ITypeSymbol? type)
         {
             var current = type;
             while (current != null)
@@ -424,7 +428,7 @@ public static bool IsAttribute(this ITypeSymbol symbol)
             return false;
         }
 
-        public static bool IsFormattableString(this ITypeSymbol symbol)
+        public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             return symbol?.MetadataName == "FormattableString"
                 && symbol.ContainingType == null
@@ -432,31 +436,31 @@ public static bool IsFormattableString(this ITypeSymbol symbol)
                 && symbol.ContainingNamespace.ContainingNamespace?.IsGlobalNamespace == true;
         }
 
-        public static ITypeSymbol RemoveUnavailableTypeParameters(
-            this ITypeSymbol type,
+        public static ITypeSymbol? RemoveUnavailableTypeParameters(
+            this ITypeSymbol? type,
             Compilation compilation,
             IEnumerable<ITypeParameterSymbol> availableTypeParameters)
         {
             return type?.RemoveUnavailableTypeParameters(compilation, availableTypeParameters.Select(t => t.Name).ToSet());
         }
 
-        private static ITypeSymbol RemoveUnavailableTypeParameters(
-            this ITypeSymbol type,
+        private static ITypeSymbol? RemoveUnavailableTypeParameters(
+            this ITypeSymbol? type,
             Compilation compilation,
             ISet<string> availableTypeParameterNames)
         {
             return type?.Accept(new UnavailableTypeParameterRemover(compilation, availableTypeParameterNames));
         }
 
-        public static ITypeSymbol RemoveAnonymousTypes(
-            this ITypeSymbol type,
+        public static ITypeSymbol? RemoveAnonymousTypes(
+            this ITypeSymbol? type,
             Compilation compilation)
         {
             return type?.Accept(new AnonymousTypeRemover(compilation));
         }
 
-        public static ITypeSymbol ReplaceTypeParametersBasedOnTypeConstraints(
-            this ITypeSymbol type,
+        public static ITypeSymbol? ReplaceTypeParametersBasedOnTypeConstraints(
+            this ITypeSymbol? type,
             Compilation compilation,
             IEnumerable<ITypeParameterSymbol> availableTypeParameters,
             Solution solution,
@@ -465,15 +469,15 @@ public static bool IsFormattableString(this ITypeSymbol symbol)
             return type?.Accept(new ReplaceTypeParameterBasedOnTypeConstraintVisitor(compilation, availableTypeParameters.Select(t => t.Name).ToSet(), solution, cancellationToken));
         }
 
-        public static ITypeSymbol RemoveUnnamedErrorTypes(
-            this ITypeSymbol type,
+        public static ITypeSymbol? RemoveUnnamedErrorTypes(
+            this ITypeSymbol? type,
             Compilation compilation)
         {
             return type?.Accept(new UnnamedErrorTypeRemover(compilation));
         }
 
         public static IList<ITypeParameterSymbol> GetReferencedMethodTypeParameters(
-            this ITypeSymbol type, IList<ITypeParameterSymbol> result = null)
+            this ITypeSymbol? type, IList<ITypeParameterSymbol>? result = null)
         {
             result ??= new List<ITypeParameterSymbol>();
             type?.Accept(new CollectTypeParameterSymbolsVisitor(result, onlyMethodTypeParameters: true));
@@ -481,15 +485,15 @@ public static bool IsFormattableString(this ITypeSymbol symbol)
         }
 
         public static IList<ITypeParameterSymbol> GetReferencedTypeParameters(
-            this ITypeSymbol type, IList<ITypeParameterSymbol> result = null)
+            this ITypeSymbol? type, IList<ITypeParameterSymbol>? result = null)
         {
             result ??= new List<ITypeParameterSymbol>();
             type?.Accept(new CollectTypeParameterSymbolsVisitor(result, onlyMethodTypeParameters: false));
             return result;
         }
 
-        public static ITypeSymbol SubstituteTypes<TType1, TType2>(
-            this ITypeSymbol type,
+        public static ITypeSymbol? SubstituteTypes<TType1, TType2>(
+            this ITypeSymbol? type,
             IDictionary<TType1, TType2> mapping,
             Compilation compilation)
             where TType1 : ITypeSymbol
@@ -498,8 +502,8 @@ public static bool IsFormattableString(this ITypeSymbol symbol)
             return type.SubstituteTypes(mapping, new CompilationTypeGenerator(compilation));
         }
 
-        public static ITypeSymbol SubstituteTypes<TType1, TType2>(
-            this ITypeSymbol type,
+        public static ITypeSymbol? SubstituteTypes<TType1, TType2>(
+            this ITypeSymbol? type,
             IDictionary<TType1, TType2> mapping,
             ITypeGenerator typeGenerator)
             where TType1 : ITypeSymbol
@@ -535,7 +539,7 @@ public static bool IsUnexpressibleTypeParameterConstraint(this ITypeSymbol typeS
             return false;
         }
 
-        public static bool IsNumericType(this ITypeSymbol type)
+        public static bool IsNumericType([NotNullWhen(returnValue: true)] this ITypeSymbol? type)
         {
             if (type != null)
             {
@@ -564,7 +568,7 @@ public static Accessibility DetermineMinimalAccessibility(this ITypeSymbol typeS
             return typeSymbol.Accept(MinimalAccessibilityVisitor.Instance);
         }
 
-        public static bool ContainsAnonymousType(this ITypeSymbol symbol)
+        public static bool ContainsAnonymousType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             switch (symbol)
             {
@@ -614,7 +618,7 @@ public static string CreateParameterName(this ITypeSymbol type, bool capitalize
             return capitalize ? shortName.ToPascalCase() : shortName.ToCamelCase();
         }
 
-        private static string GetParameterName(ITypeSymbol type)
+        private static string GetParameterName(ITypeSymbol? type)
         {
             if (type == null || type.IsAnonymousType() || type.IsTupleType)
             {
@@ -632,7 +636,7 @@ private static string GetParameterName(ITypeSymbol type)
                 : shortName;
         }
 
-        public static bool IsSpecialType(this ITypeSymbol symbol)
+        public static bool IsSpecialType([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
         {
             if (symbol != null)
             {
@@ -672,7 +676,7 @@ public static bool CanSupportCollectionInitializer(this ITypeSymbol typeSymbol,
                     .Any(m => m.Parameters.Any());
         }
 
-        public static INamedTypeSymbol GetDelegateType(this ITypeSymbol typeSymbol, Compilation compilation)
+        public static INamedTypeSymbol? GetDelegateType(this ITypeSymbol? typeSymbol, Compilation compilation)
         {
             if (typeSymbol != null)
             {
@@ -703,7 +707,7 @@ public static INamedTypeSymbol GetDelegateType(this ITypeSymbol typeSymbol, Comp
             return types.SelectMany(x => x.GetMembers().OfType<T>().Where(m => m.IsAccessibleWithin(within)));
         }
 
-        public static ImmutableArray<T> GetAccessibleMembersInThisAndBaseTypes<T>(this ITypeSymbol containingType, ISymbol within) where T : class, ISymbol
+        public static ImmutableArray<T> GetAccessibleMembersInThisAndBaseTypes<T>(this ITypeSymbol? containingType, ISymbol within) where T : class, ISymbol
         {
             if (containingType == null)
             {
@@ -748,7 +752,7 @@ public static INamedTypeSymbol GetDelegateType(this ITypeSymbol typeSymbol, Comp
             return result;
         }
 
-        private static IEnumerable<T> SelectAccessibleMembers<T>(this IEnumerable<ITypeSymbol> types, ISymbol within) where T : class, ISymbol
+        private static IEnumerable<T> SelectAccessibleMembers<T>(this IEnumerable<ITypeSymbol>? types, ISymbol within) where T : class, ISymbol
         {
             if (types == null)
             {
@@ -758,7 +762,7 @@ public static INamedTypeSymbol GetDelegateType(this ITypeSymbol typeSymbol, Comp
             return types.SelectMany(x => x.GetMembers().OfType<T>().Where(m => m.IsAccessibleWithin(within)));
         }
 
-        private static IEnumerable<T> SelectAccessibleMembers<T>(this IEnumerable<ITypeSymbol> types, string memberName, ISymbol within) where T : class, ISymbol
+        private static IEnumerable<T> SelectAccessibleMembers<T>(this IEnumerable<ITypeSymbol>? types, string memberName, ISymbol within) where T : class, ISymbol
         {
             if (types == null)
             {
@@ -848,7 +852,7 @@ public static INamedTypeSymbol GetDelegateType(this ITypeSymbol typeSymbol, Comp
             return allTypeArgs1.AreMoreSpecificThan(allTypeArgs2);
         }
 
-        public static bool IsOrDerivesFromExceptionType(this ITypeSymbol type, Compilation compilation)
+        public static bool IsOrDerivesFromExceptionType([NotNullWhen(returnValue: true)] this ITypeSymbol? type, Compilation compilation)
         {
             if (type != null)
             {
@@ -965,7 +969,7 @@ public static bool IsEnumType(this ITypeSymbol type)
             return false;
         }
 
-        public static bool IsDisposable(this ITypeSymbol type, ITypeSymbol iDisposableType)
+        public static bool IsDisposable([NotNullWhen(returnValue: true)] this ITypeSymbol? type, [NotNullWhen(returnValue: true)] ITypeSymbol? iDisposableType)
             => iDisposableType != null &&
                (Equals(iDisposableType, type) ||
                 type?.AllInterfaces.Contains(iDisposableType) == true);

src/Workspaces/Core/Portable/Shared/Extensions/SymbolInfoExtensions.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.
 
+#nullable enable
+
 using System.Collections.Immutable;
 using System.Linq;
 using Microsoft.CodeAnalysis.PooledObjects;
@@ -29,7 +31,7 @@ private static ImmutableArray<ISymbol> GetAllSymbolsWorker(this SymbolInfo info)
             }
         }
 
-        public static ISymbol GetAnySymbol(this SymbolInfo info)
+        public static ISymbol? GetAnySymbol(this SymbolInfo info)
         {
             return info.Symbol != null
                 ? info.Symbol