Fix for #9463: iterator rewriter to report missing types gracefully

src/Compilers/CSharp/Portable/Binder/Binder_Symbols.cs

@@ -990,7 +990,12 @@ private bool ShouldCheckConstraints
 
         internal NamedTypeSymbol GetSpecialType(SpecialType typeId, DiagnosticBag diagnostics, CSharpSyntaxNode node)
         {
-            NamedTypeSymbol typeSymbol = this.Compilation.GetSpecialType(typeId);
+            return GetSpecialType(this.Compilation, typeId, node, diagnostics);
+        }
+
+        internal static NamedTypeSymbol GetSpecialType(CSharpCompilation compilation, SpecialType typeId, CSharpSyntaxNode node, DiagnosticBag diagnostics)
+        {
+            NamedTypeSymbol typeSymbol = compilation.GetSpecialType(typeId);
             Debug.Assert((object)typeSymbol != null, "Expect an error type if special type isn't found");
             ReportUseSiteDiagnostics(typeSymbol, diagnostics, node);
             return typeSymbol;

src/Compilers/CSharp/Portable/Lowering/IteratorRewriter/IteratorRewriter.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.
+// 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;
 using System.Collections.Immutable;
 using System.Diagnostics;
 using Microsoft.CodeAnalysis.CodeGen;
@@ -74,7 +75,88 @@ internal partial class IteratorRewriter : StateMachineRewriter
 
             stateMachineType = new IteratorStateMachine(slotAllocatorOpt, compilationState, method, methodOrdinal, isEnumerable, elementType);
             compilationState.ModuleBuilderOpt.CompilationState.SetStateMachineType(method, stateMachineType);
-            return new IteratorRewriter(body, method, isEnumerable, stateMachineType, slotAllocatorOpt, compilationState, diagnostics).Rewrite();
+            var rewriter = new IteratorRewriter(body, method, isEnumerable, stateMachineType, slotAllocatorOpt, compilationState, diagnostics);
+            if (!rewriter.VerifyPresenceOfRequiredAPIs())
+            {
+                return body;
+            }
+
+            return rewriter.Rewrite();
+        }
+
+        /// <returns>
+        /// Returns true if all types and members we need are present and good
+        /// </returns>
+        protected bool VerifyPresenceOfRequiredAPIs()
+        {
+            DiagnosticBag bag = DiagnosticBag.GetInstance();
+
+            EnsureSpecialType(SpecialType.System_Int32, bag);
+            EnsureSpecialType(SpecialType.System_IDisposable, bag);
+            EnsureSpecialMember(SpecialMember.System_IDisposable__Dispose, bag);
+
+            // IEnumerator
+            EnsureSpecialType(SpecialType.System_Collections_IEnumerator, bag);
+            EnsureSpecialPropertyGetter(SpecialMember.System_Collections_IEnumerator__Current, bag);
+            EnsureSpecialMember(SpecialMember.System_Collections_IEnumerator__MoveNext, bag);
+            EnsureSpecialMember(SpecialMember.System_Collections_IEnumerator__Reset, bag);
+
+            // IEnumerator<T>
+            EnsureSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T, bag);
+            EnsureSpecialPropertyGetter(SpecialMember.System_Collections_Generic_IEnumerator_T__Current, bag);
+
+            if (_isEnumerable)
+            {
+                // IEnumerable and IEnumerable<T>
+                EnsureSpecialType(SpecialType.System_Collections_IEnumerable, bag);
+                EnsureSpecialMember(SpecialMember.System_Collections_IEnumerable__GetEnumerator, bag);
+                EnsureSpecialType(SpecialType.System_Collections_Generic_IEnumerable_T, bag);
+                EnsureSpecialMember(SpecialMember.System_Collections_Generic_IEnumerable_T__GetEnumerator, bag);
+            }
+
+            bool hasErrors = bag.HasAnyErrors();
+            if (hasErrors)
+            {
+                diagnostics.AddRange(bag);
+            }
+
+            bag.Free();
+            return !hasErrors;
+        }
+
+        private Symbol EnsureSpecialMember(SpecialMember member, DiagnosticBag bag)
+        {
+            Symbol symbol;
+            Binder.TryGetSpecialTypeMember(F.Compilation, member, body.Syntax, bag, out symbol);
+            return symbol;
+        }
+
+        private void EnsureSpecialType(SpecialType type, DiagnosticBag bag)
+        {
+            Binder.GetSpecialType(F.Compilation, type, body.Syntax, bag);
+        }
+
+        /// <summary>
+        /// Check that the property and its getter exist and collect any use-site errors.
+        /// </summary>
+        private void EnsureSpecialPropertyGetter(SpecialMember member, DiagnosticBag bag)
+        {
+            PropertySymbol symbol = (PropertySymbol)EnsureSpecialMember(member, bag);
+            if ((object)symbol != null)
+            {
+                var getter = symbol.GetMethod;
+                if ((object)getter == null)
+                {
+                    Binder.Error(bag, ErrorCode.ERR_PropertyLacksGet, body.Syntax, symbol);
+                    return;
+                }
+
+                var info = getter.GetUseSiteDiagnostic();
+                if ((object)info != null)
+                {
+                    bag.Add(new CSDiagnostic(info, body.Syntax.Location));
+                }
+            }
         }
 
         protected override bool PreserveInitialParameterValues

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenIterators.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.
+// 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;
 using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
+using Microsoft.CodeAnalysis.Test.Utilities;
 using Roslyn.Test.Utilities;
+using System;
 using Xunit;
 
 namespace Microsoft.CodeAnalysis.CSharp.UnitTests.CodeGen
@@ -2343,7 +2344,6 @@ public class NotSupportedException : Exception { }
     public class ValueType { }
     public class Enum { }
     public struct Void { }
-
     public interface IDisposable
     {
         void Dispose();
@@ -2394,5 +2394,132 @@ public System.Collections.IEnumerable SomeNumbers()
                 // warning CS8021: No value for RuntimeMetadataVersion found. No assembly containing System.Object was found nor was a value for RuntimeMetadataVersion specified through options.
                 Diagnostic(ErrorCode.WRN_NoRuntimeMetadataVersion).WithLocation(1, 1));
         }
+
+        [Fact, WorkItem(9463, "https://github.com/dotnet/roslyn/issues/9463")]
+        public void IEnumerableIteratorReportsDiagnosticsWhenCoreTypesAreMissing()
+        {
+            // Note that IDisposable.Dispose, IEnumerator.Current and other types are missing
+            // Also, IEnumerator<T> doesn't have a get accessor
+            var source = @"
+namespace System
+{
+    public class Object { }
+    public struct Int32 { }
+    public struct Boolean { }
+    public class String { }
+    public class Exception { }
+    public class ValueType { }
+    public class Enum { }
+    public struct Void { }
+    public interface IDisposable { }
+}
+
+namespace System.Collections
+{
+    public interface IEnumerable { }
+    public interface IEnumerator { }
+}
+
+namespace System.Collections.Generic
+{
+    public interface IEnumerator<T>
+    {
+        T Current { set; }
+    }
+}
+
+public class C
+{
+    public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+}";
+            var compilation = CreateCompilation(new[] { Parse(source) });
+
+            compilation.VerifyEmitDiagnostics(
+                // warning CS8021: No value for RuntimeMetadataVersion found. No assembly containing System.Object was found nor was a value for RuntimeMetadataVersion specified through options.
+                Diagnostic(ErrorCode.WRN_NoRuntimeMetadataVersion).WithLocation(1, 1),
+                // (31,57): error CS0656: Missing compiler required member 'System.IDisposable.Dispose'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.IDisposable", "Dispose").WithLocation(31, 57),
+                // (31,57): error CS0154: The property or indexer 'IEnumerator<T>.Current' cannot be used in this context because it lacks the get accessor
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_PropertyLacksGet, "{ yield return 42; }").WithArguments("System.Collections.Generic.IEnumerator<T>.Current").WithLocation(31, 57),
+                // (31,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerator.Current'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator", "Current").WithLocation(31, 57),
+                // (31,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerator.MoveNext'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator", "MoveNext").WithLocation(31, 57),
+                // (31,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerator.Reset'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator", "Reset").WithLocation(31, 57),
+                // (31,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerable.GetEnumerator'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerable", "GetEnumerator").WithLocation(31, 57),
+                // (31,57): error CS0518: Predefined type 'System.Collections.Generic.IEnumerable`1' is not defined or imported
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "{ yield return 42; }").WithArguments("System.Collections.Generic.IEnumerable`1").WithLocation(31, 57),
+                // (31,57): error CS0656: Missing compiler required member 'System.Collections.Generic.IEnumerable`1.GetEnumerator'
+                //     public System.Collections.IEnumerable SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.Generic.IEnumerable`1", "GetEnumerator").WithLocation(31, 57));
+        }
+
+        [Fact, WorkItem(9463, "https://github.com/dotnet/roslyn/issues/9463")]
+        public void IEnumeratorIteratorReportsDiagnosticsWhenCoreTypesAreMissing()
+        {
+            // Note that IDisposable.Dispose and other types are missing
+            // Also IEnumerator.Current lacks a get accessor
+            var source = @"
+namespace System
+{
+    public class Object { }
+    public struct Int32 { }
+    public struct Boolean { }
+    public class String { }
+    public class Exception { }
+    public class ValueType { }
+    public class Enum { }
+    public struct Void { }
+
+    public interface IDisposable { }
+}
+
+namespace System.Collections
+{
+    public interface IEnumerable { }
+    public interface IEnumerator
+    {
+        Object Current { set; }
+    }
+}
+
+public class C
+{
+    public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+}";
+            var compilation = CreateCompilation(new[] { Parse(source) });
+
+            // No error about IEnumerable
+            compilation.VerifyEmitDiagnostics(
+                // (27,57): error CS0656: Missing compiler required member 'System.IDisposable.Dispose'
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.IDisposable", "Dispose").WithLocation(27, 57),
+                // (27,57): error CS0518: Predefined type 'System.Collections.Generic.IEnumerator`1' is not defined or imported
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_PredefinedTypeNotFound, "{ yield return 42; }").WithArguments("System.Collections.Generic.IEnumerator`1").WithLocation(27, 57),
+                // (27,57): error CS0656: Missing compiler required member 'System.Collections.Generic.IEnumerator`1.Current'
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.Generic.IEnumerator`1", "Current").WithLocation(27, 57),
+                // (27,57): error CS0154: The property or indexer 'IEnumerator.Current' cannot be used in this context because it lacks the get accessor
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_PropertyLacksGet, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator.Current").WithLocation(27, 57),
+                // (27,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerator.MoveNext'
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator", "MoveNext").WithLocation(27, 57),
+                // (27,57): error CS0656: Missing compiler required member 'System.Collections.IEnumerator.Reset'
+                //     public System.Collections.IEnumerator SomeNumbers() { yield return 42; }
+                Diagnostic(ErrorCode.ERR_MissingPredefinedMember, "{ yield return 42; }").WithArguments("System.Collections.IEnumerator", "Reset").WithLocation(27, 57),
+                // warning CS8021: No value for RuntimeMetadataVersion found. No assembly containing System.Object was found nor was a value for RuntimeMetadataVersion specified through options.
+                Diagnostic(ErrorCode.WRN_NoRuntimeMetadataVersion).WithLocation(1, 1));
+        }
     }
 }

src/Compilers/VisualBasic/Portable/Binding/Binder.vb

@@ -414,11 +414,20 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         ''' <param name="node">Where to report the error, if any.</param>
         Public Function GetSpecialType(typeId As SpecialType, node As SyntaxNodeOrToken, diagBag As DiagnosticBag) As NamedTypeSymbol
             Dim reportedAnError As Boolean = False
-            Return GetSpecialType(typeId, node, diagBag, reportedAnError, suppressUseSiteError:=False)
+            Return GetSpecialType(SourceModule.ContainingAssembly, typeId, node, diagBag, reportedAnError, suppressUseSiteError:=False)
+        End Function
+
+        Public Shared Function GetSpecialType(assembly As AssemblySymbol, typeId As SpecialType, node As SyntaxNodeOrToken, diagBag As DiagnosticBag) As NamedTypeSymbol
+            Dim reportedAnError As Boolean = False
+            Return GetSpecialType(assembly, typeId, node, diagBag, reportedAnError, suppressUseSiteError:=False)
         End Function
 
         Public Function GetSpecialType(typeId As SpecialType, node As SyntaxNodeOrToken, diagBag As DiagnosticBag, ByRef reportedAnError As Boolean, suppressUseSiteError As Boolean) As NamedTypeSymbol
-            Dim symbol As NamedTypeSymbol = SourceModule.ContainingAssembly.GetSpecialType(typeId)
+            Return GetSpecialType(SourceModule.ContainingAssembly, typeId, node, diagBag, reportedAnError, suppressUseSiteError)
+        End Function
+
+        Public Shared Function GetSpecialType(assembly As AssemblySymbol, typeId As SpecialType, node As SyntaxNodeOrToken, diagBag As DiagnosticBag, ByRef reportedAnError As Boolean, suppressUseSiteError As Boolean) As NamedTypeSymbol
+            Dim symbol As NamedTypeSymbol = assembly.GetSpecialType(typeId)
 
             If diagBag IsNot Nothing Then
                 Dim info = GetUseSiteErrorForSpecialType(symbol, suppressUseSiteError)
@@ -526,8 +535,12 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         ''' member isn't found.
         ''' </summary>
         Friend Function GetSpecialTypeMember(member As SpecialMember, syntax As VisualBasicSyntaxNode, diagnostics As DiagnosticBag) As Symbol
+            Return GetSpecialTypeMember(Me.ContainingMember.ContainingAssembly, member, syntax, diagnostics)
+        End Function
+
+        Friend Shared Function GetSpecialTypeMember(assembly As AssemblySymbol, member As SpecialMember, syntax As VisualBasicSyntaxNode, diagnostics As DiagnosticBag) As Symbol
             Dim useSiteError As DiagnosticInfo = Nothing
-            Dim specialMemberSymbol As Symbol = GetSpecialTypeMember(Me.ContainingMember.ContainingAssembly, member, useSiteError)
+            Dim specialMemberSymbol As Symbol = GetSpecialTypeMember(assembly, member, useSiteError)
 
             If useSiteError IsNot Nothing Then
                 ReportDiagnostic(diagnostics, syntax, useSiteError)

src/Compilers/VisualBasic/Portable/Lowering/IteratorRewriter/IteratorRewriter.vb

@@ -79,38 +79,51 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             Return rewriter.Rewrite()
         End Function
 
+        ''' <returns>
+        ''' Returns true if any members that we need are missing or have use-site errors.
+        ''' </returns>
         Friend Overrides Function EnsureAllSymbolsAndSignature() As Boolean
-            Dim hasErrors As Boolean = MyBase.EnsureAllSymbolsAndSignature
-
-            If Me.Method.IsSub OrElse Me._elementType.IsErrorType Then
-                hasErrors = True
+            If MyBase.EnsureAllSymbolsAndSignature OrElse Me.Method.IsSub OrElse Me._elementType.IsErrorType Then
+                Return True
             End If
 
+            Dim bag = DiagnosticBag.GetInstance()
+
             ' NOTE: We don't ensure DebuggerStepThroughAttribute, it is just not emitted if not found
             ' EnsureWellKnownMember(Of MethodSymbol)(WellKnownMember.System_Diagnostics_DebuggerStepThroughAttribute__ctor, hasErrors)
 
             ' TODO: do we need these here? They are used on the actual iterator method.
             ' EnsureWellKnownMember(Of MethodSymbol)(WellKnownMember.System_Runtime_CompilerServices_IteratorStateMachine__ctor, hasErrors)
 
-            EnsureSpecialType(SpecialType.System_Object, hasErrors)
-            EnsureSpecialType(SpecialType.System_Boolean, hasErrors)
-            EnsureSpecialType(SpecialType.System_Int32, hasErrors)
-
-            If Me.Method.ReturnType.IsDefinition Then
-                If Me._isEnumerable Then
-                    EnsureSpecialType(SpecialType.System_Collections_IEnumerator, hasErrors)
-                End If
-            Else
-                If Me._isEnumerable Then
-                    EnsureSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T, hasErrors)
-                    EnsureSpecialType(SpecialType.System_Collections_IEnumerable, hasErrors)
-                End If
-
-                EnsureSpecialType(SpecialType.System_Collections_IEnumerator, hasErrors)
+            EnsureSpecialType(SpecialType.System_Object, bag)
+            EnsureSpecialType(SpecialType.System_Boolean, bag)
+            EnsureSpecialType(SpecialType.System_Int32, bag)
+            EnsureSpecialType(SpecialType.System_IDisposable, bag)
+            EnsureSpecialMember(SpecialMember.System_IDisposable__Dispose, bag)
+
+            ' IEnumerator
+            EnsureSpecialType(SpecialType.System_Collections_IEnumerator, bag)
+            EnsureSpecialPropertyGetter(SpecialMember.System_Collections_IEnumerator__Current, bag)
+            EnsureSpecialMember(SpecialMember.System_Collections_IEnumerator__MoveNext, bag)
+            EnsureSpecialMember(SpecialMember.System_Collections_IEnumerator__Reset, bag)
+
+            ' IEnumerator(Of T)
+            EnsureSpecialType(SpecialType.System_Collections_Generic_IEnumerator_T, bag)
+            EnsureSpecialPropertyGetter(SpecialMember.System_Collections_Generic_IEnumerator_T__Current, bag)
+
+            If Me._isEnumerable Then
+                EnsureSpecialType(SpecialType.System_Collections_IEnumerable, bag)
+                EnsureSpecialMember(SpecialMember.System_Collections_IEnumerable__GetEnumerator, bag)
+                EnsureSpecialType(SpecialType.System_Collections_Generic_IEnumerable_T, bag)
+                EnsureSpecialMember(SpecialMember.System_Collections_Generic_IEnumerable_T__GetEnumerator, bag)
             End If
 
-            EnsureSpecialType(SpecialType.System_IDisposable, hasErrors)
+            Dim hasErrors As Boolean = bag.HasAnyErrors
+            If hasErrors Then
+                Me.Diagnostics.AddRange(bag)
+            End If
 
+            bag.Free()
             Return hasErrors
         End Function
 

src/Compilers/VisualBasic/Portable/Lowering/StateMachineRewriter/StateMachineRewriter.vb

@@ -368,6 +368,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             Return False
         End Function
 
+        Friend Function EnsureSpecialType(type As SpecialType, bag As DiagnosticBag) As Symbol
+            Return Binder.GetSpecialType(F.Compilation.Assembly, type, Me.Body.Syntax, bag)
+        End Function
+
         Friend Sub EnsureSpecialType(type As SpecialType, <[In], Out> ByRef hasErrors As Boolean)
             Dim sType = Me.F.SpecialType(type)
             If sType.GetUseSiteErrorInfo IsNot Nothing Then
@@ -375,6 +379,31 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
             End If
         End Sub
 
+        Friend Function EnsureSpecialMember(member As SpecialMember, bag As DiagnosticBag) As Symbol
+            Return Binder.GetSpecialTypeMember(F.Compilation.Assembly, member, Me.Body.Syntax, bag)
+        End Function
+
+        ''' <summary>
+        ''' Check that the property and its getter exist and collect any use-site errors.
+        ''' </summary>
+        Friend Sub EnsureSpecialPropertyGetter(member As SpecialMember, bag As DiagnosticBag)
+            Dim symbol = DirectCast(EnsureSpecialMember(member, bag), PropertySymbol)
+
+            If symbol IsNot Nothing Then
+                Dim getter = symbol.GetMethod
+
+                If getter Is Nothing Then
+                    Binder.ReportDiagnostic(bag, Body.Syntax, ERRID.ERR_NoGetProperty1, CustomSymbolDisplayFormatter.QualifiedName(symbol))
+                    Return
+                End If
+
+                Dim useSiteError = getter.GetUseSiteErrorInfo()
+                If useSiteError IsNot Nothing Then
+                    Binder.ReportDiagnostic(bag, Body.Syntax, useSiteError)
+                End If
+            End If
+        End Sub
+
         Friend Sub EnsureWellKnownType(type As WellKnownType, <[In], Out> ByRef hasErrors As Boolean)
             Dim wkType = Me.F.WellKnownType(type)
             If wkType.GetUseSiteErrorInfo IsNot Nothing Then

src/Compilers/VisualBasic/Test/Emit/CodeGen/CodeGenIterators.vb

-' Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+' Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+Imports Microsoft.CodeAnalysis.Test.Utilities
 Imports Roslyn.Test.Utilities
 
 Namespace Microsoft.CodeAnalysis.VisualBasic.UnitTests
@@ -1688,5 +1689,146 @@ End Class
             Dim verifier = CompileAndVerify(compilation, verify:=False)
             verifier.VerifyDiagnostics()
         End Sub
+
+        <Fact, WorkItem(9463, "https://github.com/dotnet/roslyn/issues/9463")>
+        Public Sub IEnumerableIteratorReportsDiagnosticsWhenCoreTypesAreMissing()
+            ' Note that IDisposable.Dispose, IEnumerator.Current and other types are missing
+            ' Also, IEnumerator(Of T) doesn't have a get accessor
+            Dim source = "
+Namespace System
+    Public Class [Object]
+    End Class
+    Public Class [Int32]
+    End Class
+    Public Class [Boolean]
+    End Class
+    Public Class [String]
+    End Class
+    Public Class Exception
+    End Class
+    Public Class ValueType
+    End Class
+    Public Class [Enum]
+    End Class
+    Public Class Void
+    End Class
+    Public Interface IDisposable
+    End Interface
+End Namespace
+
+Namespace System.Collections
+    Public Interface IEnumerable
+    End Interface
+    Public Interface IEnumerator
+    End Interface
+End Namespace
+
+Namespace System.Collections.Generic
+    Public Interface IEnumerator(Of T)
+        WriteOnly Property Current As T
+    End Interface
+End Namespace
+
+Class C
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+        Yield Nothing
+    End Function
+End Class
+"
+            Dim compilation = CreateCompilation({Parse(source)})
+
+            compilation.AssertTheseEmitDiagnostics(<expected>
+BC30002: Type 'System.Collections.Generic.IEnumerable' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC30524: Property 'System.Collections.Generic.IEnumerator(Of T).Current' is 'WriteOnly'.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.Generic.IEnumerable`1.GetEnumerator' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerable.GetEnumerator' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerator.Current' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerator.MoveNext' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerator.Reset' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.IDisposable.Dispose' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerable
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+                </expected>)
+        End Sub
+
+        <Fact, WorkItem(9463, "https://github.com/dotnet/roslyn/issues/9463")>
+        Public Sub IEnumeratorIteratorReportsDiagnosticsWhenCoreTypesAreMissing()
+            ' Note that IDisposable.Dispose and other types are missing
+            ' Also IEnumerator.Current lacks a get accessor
+            Dim source = "
+Namespace System
+    Public Class [Object]
+    End Class
+    Public Class [Int32]
+    End Class
+    Public Class [Boolean]
+    End Class
+    Public Class [String]
+    End Class
+    Public Class Exception
+    End Class
+    Public Class ValueType
+    End Class
+    Public Class [Enum]
+    End Class
+    Public Class Void
+    End Class
+    Public Interface IDisposable
+    End Interface
+End Namespace
+
+Namespace System.Collections
+    Public Interface IEnumerable
+    End Interface
+    Public Interface IEnumerator
+        WriteOnly Property Current As Object
+    End Interface
+End Namespace
+
+Class C
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+        Yield Nothing
+    End Function
+End Class
+"
+            Dim compilation = CreateCompilation({Parse(source)})
+
+            ' No error about IEnumerable
+            compilation.AssertTheseEmitDiagnostics(<expected>
+BC30002: Type 'System.Collections.Generic.IEnumerator' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC30524: Property 'System.Collections.IEnumerator.Current' is 'WriteOnly'.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.Generic.IEnumerator`1.Current' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerator.MoveNext' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.Collections.IEnumerator.Reset' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+BC35000: Requested operation is not available because the runtime library function 'System.IDisposable.Dispose' is not defined.
+    Public Iterator Function SomeNumbers() As System.Collections.IEnumerator
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+                </expected>)
+        End Sub
+
     End Class
 End Namespace

src/Test/Utilities/Shared/Assert/AssertEx.cs

@@ -441,10 +441,13 @@ public static string GetAssertMessage<T>(IEnumerable<T> expected, IEnumerable<T>
                 }
             }
 
+            var expectedString = string.Join(itemSeparator, expected.Select(itemInspector));
             var actualString = string.Join(itemSeparator, actual.Select(itemInspector));
 
             var message = new StringBuilder();
             message.AppendLine();
+            message.AppendLine("Expected:");
+            message.AppendLine(expectedString);
             message.AppendLine("Actual:");
             message.AppendLine(actualString);
             message.AppendLine("Differences:");