addressed some feedback on IsInvalid and some other decision we made on design meeting.

IsInvalid -> HasDiagnostics
removed HasChildren interface and merged it to IOperation

src/Compilers/CSharp/Test/Semantic/IOperation/IOperationTests_IArgument.cs

@@ -1885,7 +1885,7 @@ public static void Verify(IOperation operation, Compilation compilation, SyntaxN
 
             public override void VisitPropertyReferenceExpression(IPropertyReferenceExpression operation)
             {
-                if (operation.IsInvalid(_compilation) || operation.ArgumentsInEvaluationOrder.Length == 0)
+                if (operation.HasDiagnostics(_compilation) || operation.ArgumentsInEvaluationOrder.Length == 0)
                 {
                     return;
                 }
@@ -1894,7 +1894,7 @@ public override void VisitPropertyReferenceExpression(IPropertyReferenceExpressi
                 var indexerSymbol = operation.Property;
                 foreach (var argument in operation.ArgumentsInEvaluationOrder)
                 {
-                    if (!argument.IsInvalid(_compilation))
+                    if (!argument.HasDiagnostics(_compilation))
                     {
                         Assert.Same(indexerSymbol, argument.Parameter.ContainingSymbol);
                     }

src/Compilers/Core/Portable/Operations/IOperation.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.
 
+using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 using Microsoft.CodeAnalysis.Semantics;
 
@@ -35,6 +36,11 @@ public interface IOperation
         /// </summary>
         Optional<object> ConstantValue { get; }
 
+        /// <summary>
+        /// An array of child operations for this operation.
+        /// </summary>
+        IEnumerable<IOperation> Children { get; }
+
         void Accept(OperationVisitor visitor);
 
         TResult Accept<TArgument, TResult>(OperationVisitor<TArgument, TResult> visitor, TArgument argument);

src/Compilers/Core/Portable/Operations/IOperationWithChildren.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.
-
-using System.Collections.Generic;
-using System.Runtime.CompilerServices;
-
-namespace Microsoft.CodeAnalysis
-{
-    /// <summary>
-    /// Root type for <see cref="IOperation"/> nodes that are still not fully designed and hence need a backdoor to stitch the child IOperation nodes to the entire IOperation tree.
-    /// </summary>
-    /// <remarks>
-    /// NOTE: This type is a temporary workaround and should be deleted once we ship IOperation APIs.
-    /// </remarks>
-    [InternalImplementationOnly]
-    internal interface IOperationWithChildren: IOperation
-    {
-        /// <summary>
-        /// An array of child operations for this operation.
-        /// </summary>
-        /// <remarks>Note that any of the child operation nodes may be null.</remarks>
-        IEnumerable<IOperation> Children { get; }
-    }
-}

src/Compilers/Core/Portable/Operations/Operation.cs

@@ -10,7 +10,7 @@ namespace Microsoft.CodeAnalysis
     /// <summary>
     /// Root type for representing the abstract semantics of C# and VB statements and expressions.
     /// </summary>
-    internal abstract class Operation : IOperation, IOperationWithChildren
+    internal abstract class Operation : IOperation
     {
         public Operation(OperationKind kind, SyntaxNode syntax, ITypeSymbol type, Optional<object> constantValue)
         {
@@ -51,7 +51,7 @@ public static IOperation CreateOperationNone(SyntaxNode node, Optional<object> c
             return new NoneOperation(node, constantValue, getChildren);
         }
 
-        private class NoneOperation : IOperation, IOperationWithChildren
+        private class NoneOperation : IOperation
         {
             private readonly Func<ImmutableArray<IOperation>> _getChildren;
 

src/Compilers/Core/Portable/Operations/OperationExtensions.cs

@@ -14,7 +14,7 @@ public static class OperationExtensions
         /// <summary>
         /// This will check whether context around the operation has any error such as syntax or semantic error
         /// </summary>
-        public static bool IsInvalid(this IOperation operation, Compilation compilation, CancellationToken cancellationToken = default(CancellationToken))
+        public static bool HasDiagnostics(this IOperation operation, Compilation compilation, CancellationToken cancellationToken = default(CancellationToken))
         {
             // once we made sure every operation has Syntax, we will remove this condition
             if (operation.Syntax == null)
@@ -24,7 +24,7 @@ public static bool IsInvalid(this IOperation operation, Compilation compilation,
 
             // if wrong compilation is given, GetSemanticModel will throw due to tree not belong to the given compilation.
             var model = compilation.GetSemanticModel(operation.Syntax.SyntaxTree);
-            return model.GetDiagnostics(operation.Syntax.Span, cancellationToken).Any(d => d.Severity == DiagnosticSeverity.Error);
+            return model.GetDiagnostics(operation.Syntax.Span, cancellationToken).Any(d => d.DefaultSeverity == DiagnosticSeverity.Error);
         }
 
         public static IEnumerable<IOperation> Descendants(this IOperation operation)

src/Compilers/Core/Portable/Operations/OperationWalker.cs

@@ -12,9 +12,9 @@ public abstract class OperationWalker : OperationVisitor
     {
         private int _recursionDepth;
 
-        internal void VisitArray<T>(IEnumerable<T> list) where T : IOperation
+        internal void VisitArray<T>(IEnumerable<T> operations) where T : IOperation
         {
-            foreach (var operation in list)
+            foreach (var operation in operations)
             {
                 Visit(operation);
             }
@@ -39,18 +39,12 @@ public override void Visit(IOperation operation)
 
         public override void DefaultVisit(IOperation operation)
         {
-            if (operation is IOperationWithChildren operationWithChildren)
-            {
-                VisitArray(operationWithChildren.Children);
-            }
+            VisitArray(operation.Children);
         }
 
         internal override void VisitNoneOperation(IOperation operation)
         {
-            if (operation is IOperationWithChildren operationWithChildren)
-            {
-                VisitArray(operationWithChildren.Children);
-            }
+            VisitArray(operation.Children);
         }
     }
 }

src/Compilers/Core/Portable/PublicAPI.Unshipped.txt

 *REMOVED*Microsoft.CodeAnalysis.Compilation.Emit(System.IO.Stream peStream, System.IO.Stream pdbStream = null, System.IO.Stream xmlDocumentationStream = null, System.IO.Stream win32Resources = null, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.ResourceDescription> manifestResources = null, Microsoft.CodeAnalysis.Emit.EmitOptions options = null, Microsoft.CodeAnalysis.IMethodSymbol debugEntryPoint = null, System.IO.Stream sourceLinkStream = null, System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.EmbeddedText> embeddedTexts = null, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> Microsoft.CodeAnalysis.Emit.EmitResult
 *REMOVED*Microsoft.CodeAnalysis.Emit.EmitOptions.EmitOptions(bool metadataOnly = false, Microsoft.CodeAnalysis.Emit.DebugInformationFormat debugInformationFormat = (Microsoft.CodeAnalysis.Emit.DebugInformationFormat)0, string pdbFilePath = null, string outputNameOverride = null, int fileAlignment = 0, ulong baseAddress = 0, bool highEntropyVirtualAddressSpace = false, Microsoft.CodeAnalysis.SubsystemVersion subsystemVersion = default(Microsoft.CodeAnalysis.SubsystemVersion), string runtimeMetadataVersion = null, bool tolerateErrors = false, bool includePrivateMembers = false, System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.Emit.InstrumentationKind> instrumentationKinds = default(System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.Emit.InstrumentationKind>)) -> void
 *REMOVED*Microsoft.CodeAnalysis.IOperation.IsInvalid.get -> bool
-*REMOVED*Microsoft.CodeAnalysis.Semantics.IPropertyReferenceExpression.Property.get -> Microsoft.CodeAnalysis.IPropertySymbol
+Microsoft.CodeAnalysis.IOperation.Children.get -> System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.IOperation>
 Microsoft.CodeAnalysis.CommandLineArguments.DisplayLangVersions.get -> bool
 Microsoft.CodeAnalysis.CommandLineArguments.OutputRefFilePath.get -> string
 Microsoft.CodeAnalysis.CommandLineArguments.RuleSetPath.get -> string
@@ -738,7 +738,7 @@ static Microsoft.CodeAnalysis.Semantics.OperationExtensions.Descendants(this Mic
 static Microsoft.CodeAnalysis.Semantics.OperationExtensions.DescendantsAndSelf(this Microsoft.CodeAnalysis.IOperation operation) -> System.Collections.Generic.IEnumerable<Microsoft.CodeAnalysis.IOperation>
 static Microsoft.CodeAnalysis.Semantics.OperationExtensions.GetDeclaredVariables(this Microsoft.CodeAnalysis.Semantics.IVariableDeclarationStatement declarationStatement) -> System.Collections.Immutable.ImmutableArray<Microsoft.CodeAnalysis.ILocalSymbol>
 static Microsoft.CodeAnalysis.Semantics.OperationExtensions.GetRootOperation(this Microsoft.CodeAnalysis.ISymbol symbol, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> Microsoft.CodeAnalysis.IOperation
-static Microsoft.CodeAnalysis.Semantics.OperationExtensions.IsInvalid(this Microsoft.CodeAnalysis.IOperation operation, Microsoft.CodeAnalysis.Compilation compilation, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> bool
+static Microsoft.CodeAnalysis.Semantics.OperationExtensions.HasDiagnostics(this Microsoft.CodeAnalysis.IOperation operation, Microsoft.CodeAnalysis.Compilation compilation, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken)) -> bool
 static Microsoft.CodeAnalysis.Semantics.UnaryAndBinaryOperationExtensions.GetBinaryOperandsKind(Microsoft.CodeAnalysis.Semantics.BinaryOperationKind kind) -> Microsoft.CodeAnalysis.Semantics.BinaryOperandsKind
 static Microsoft.CodeAnalysis.Semantics.UnaryAndBinaryOperationExtensions.GetBinaryOperandsKind(this Microsoft.CodeAnalysis.Semantics.IBinaryOperatorExpression binary) -> Microsoft.CodeAnalysis.Semantics.BinaryOperandsKind
 static Microsoft.CodeAnalysis.Semantics.UnaryAndBinaryOperationExtensions.GetBinaryOperandsKind(this Microsoft.CodeAnalysis.Semantics.ICompoundAssignmentExpression compoundAssignment) -> Microsoft.CodeAnalysis.Semantics.BinaryOperandsKind

src/Test/Utilities/Portable/Compilation/OperationTreeVerifier.cs

@@ -82,7 +82,7 @@ private void LogCommonPropertiesAndNewLine(IOperation operation)
             }
 
             // IsInvalid
-            if (operation.IsInvalid(_compilation))
+            if (operation.HasDiagnostics(_compilation))
             {
                 LogString(", IsInvalid");
             }
@@ -260,13 +260,10 @@ internal override void VisitNoneOperation(IOperation operation)
             LogString("IOperation: ");
             LogCommonPropertiesAndNewLine(operation);
 
-            if (operation is IOperationWithChildren operationWithChildren)
+            var children = operation.Children;
+            if (children.Count() > 0)
             {
-                var children = operationWithChildren.Children.WhereNotNull().ToImmutableArray();
-                if (children.Length > 0)
-                {
-                    VisitArray(children, "Children", logElementCount: true);
-                }
+                VisitArray(children, "Children", logElementCount: true);
             }
         }
 

src/Test/Utilities/Portable/Diagnostics/OperationTestAnalyzer.cs

@@ -63,7 +63,7 @@ public sealed override void Initialize(AnalysisContext context)
             context.RegisterOperationAction(
                  (operationContext) =>
                  {
-                     if (operationContext.Operation.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                     if (operationContext.Operation.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                      {
                          operationContext.ReportDiagnostic(Diagnostic.Create(IsInvalidDescriptor, operationContext.Operation.Syntax.GetLocation()));
                      }
@@ -555,7 +555,7 @@ public sealed override void Initialize(AnalysisContext context)
                      long priorArgumentValue = long.MinValue;
                      foreach (IArgument argument in invocation.ArgumentsInEvaluationOrder)
                      {
-                         if (argument.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                         if (argument.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                          {
                              operationContext.ReportDiagnostic(Diagnostic.Create(InvalidArgumentDescriptor, argument.Syntax.GetLocation()));
                              return;
@@ -859,7 +859,7 @@ public sealed override void Initialize(AnalysisContext context)
 
                      foreach (var decl in declarationStatement.Declarations)
                      {
-                         if (decl.Initializer != null && !decl.Initializer.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                         if (decl.Initializer != null && !decl.Initializer.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                          {
                              foreach (var symbol in decl.Variables)
                              {
@@ -924,7 +924,7 @@ public sealed override void Initialize(AnalysisContext context)
                              break;
                          case OperationKind.SwitchCase:
                              var switchSection = (ISwitchCase)operationContext.Operation;
-                             if (!switchSection.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) && switchSection.Clauses.Length > 1)
+                             if (!switchSection.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) && switchSection.Clauses.Length > 1)
                              {
                                  Report(operationContext, switchSection.Syntax, MultipleCaseClausesDescriptor);
                              }
@@ -1062,7 +1062,7 @@ public sealed override void Initialize(AnalysisContext context)
 
                      if (eventAssignment.Event == null)
                      {
-                         if (eventAssignment.EventInstance == null && eventAssignment.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                         if (eventAssignment.EventInstance == null && eventAssignment.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                          {
                              // report inside after checking for null to make sure it does't crash.
                              operationContext.ReportDiagnostic(Diagnostic.Create(InvalidEventDescriptor, eventAssignment.Syntax.GetLocation()));
@@ -1286,7 +1286,7 @@ public sealed override void Initialize(AnalysisContext context)
                      var addressOfOperation = (IAddressOfExpression)operationContext.Operation;
                      operationContext.ReportDiagnostic(Diagnostic.Create(AddressOfDescriptor, addressOfOperation.Syntax.GetLocation()));
 
-                     if (addressOfOperation.Reference.Kind == OperationKind.InvalidExpression && addressOfOperation.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                     if (addressOfOperation.Reference.Kind == OperationKind.InvalidExpression && addressOfOperation.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                      {
                          operationContext.ReportDiagnostic(Diagnostic.Create(InvalidAddressOfReferenceDescriptor, addressOfOperation.Reference.Syntax.GetLocation()));
                      }
@@ -1674,8 +1674,8 @@ public sealed override void Initialize(AnalysisContext context)
                     var binary = (IBinaryOperatorExpression)operationContext.Operation;
                     var left = binary.LeftOperand;
                     var right = binary.RightOperand;
-                    if (!left.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) &&
-                        !right.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) &&
+                    if (!left.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) &&
+                        !right.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) &&
                         !binary.UsesOperatorMethod && binary.OperatorMethod == null)
                     {
                         if (left.Kind == OperationKind.LocalReferenceExpression)
@@ -1710,7 +1710,7 @@ public sealed override void Initialize(AnalysisContext context)
                         var operandLocal = ((ILocalReferenceExpression)operand).Local;
                         if (operandLocal.Name == "x")
                         {
-                            if (!operand.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) && !unary.UsesOperatorMethod && unary.OperatorMethod == null)
+                            if (!operand.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) && !unary.UsesOperatorMethod && unary.OperatorMethod == null)
                             {
                                 operationContext.ReportDiagnostic(
                                     Diagnostic.Create(UnaryOperatorDescriptor,
@@ -1852,7 +1852,7 @@ public sealed override void Initialize(AnalysisContext context)
                      if (operation.Kind == OperationKind.BinaryOperatorExpression)
                      {
                          var binary = (IBinaryOperatorExpression)operation;
-                         if (binary.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) && binary.BinaryOperationKind == BinaryOperationKind.Invalid)
+                         if (binary.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) && binary.BinaryOperationKind == BinaryOperationKind.Invalid)
                          {
                              operationContext.ReportDiagnostic(Diagnostic.Create(InvalidBinaryDescriptor, binary.Syntax.GetLocation()));
                          }
@@ -1860,7 +1860,7 @@ public sealed override void Initialize(AnalysisContext context)
                      else if (operation.Kind == OperationKind.UnaryOperatorExpression)
                      {
                          var unary = (IUnaryOperatorExpression)operation;
-                         if (unary.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) && unary.UnaryOperationKind == UnaryOperationKind.Invalid)
+                         if (unary.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) && unary.UnaryOperationKind == UnaryOperationKind.Invalid)
                          {
                              operationContext.ReportDiagnostic(Diagnostic.Create(InvalidUnaryDescriptor, unary.Syntax.GetLocation()));
                          }
@@ -1869,7 +1869,7 @@ public sealed override void Initialize(AnalysisContext context)
                      {
                          var inc = (IIncrementExpression)operation;
                          var binaryOperationKind = CSharpOperationFactory.Helper.DeriveBinaryOperationKind(inc.IncrementOperationKind);
-                         if (inc.IsInvalid(operationContext.Compilation) && binaryOperationKind == BinaryOperationKind.Invalid)
+                         if (inc.HasDiagnostics(operationContext.Compilation) && binaryOperationKind == BinaryOperationKind.Invalid)
                          {
                              operationContext.ReportDiagnostic(Diagnostic.Create(InvalidIncrementDescriptor, inc.Syntax.GetLocation()));
                          }
@@ -1960,7 +1960,7 @@ public sealed override void Initialize(AnalysisContext context)
                      var conversion = (IConversionExpression)operationContext.Operation;
                      if (conversion.ConversionKind == ConversionKind.Invalid)
                      {
-                         Debug.Assert(conversion.IsInvalid(operationContext.Compilation, operationContext.CancellationToken) == true);
+                         Debug.Assert(conversion.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken) == true);
                          operationContext.ReportDiagnostic(Diagnostic.Create(InvalidConversionExpressionDescriptor, conversion.Syntax.GetLocation()));
                      }
                  },
@@ -1996,7 +1996,7 @@ public sealed override void Initialize(AnalysisContext context)
                          IForLoopStatement forLoop = (IForLoopStatement)loop;
                          var forCondition = forLoop.Condition;
 
-                         if (forCondition.IsInvalid(operationContext.Compilation, operationContext.CancellationToken))
+                         if (forCondition.HasDiagnostics(operationContext.Compilation, operationContext.CancellationToken))
                          {
                              // Generate a warning to prove we didn't crash
                              operationContext.ReportDiagnostic(Diagnostic.Create(ForLoopConditionCrashDescriptor, forLoop.Condition.Syntax.GetLocation()));