Addressed PR comments

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -8999,7 +8999,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to Type &apos;{0}&apos; is forwarded to multiple assemblies: &apos;{1}&apos; and &apos;{2}&apos;.
+        ///   Looks up a localized string similar to Module &apos;{0}&apos; in assembly &apos;{1}&apos; is forwarding the type &apos;{2}&apos; to multiple assemblies: &apos;{3}&apos; and &apos;{4}&apos;..
         /// </summary>
         internal static string ERR_TypeForwardedToMultipleAssemblies {
             get {

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5009,6 +5009,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Attributes are not allowed on local function parameters or type parameters</value>
   </data>
   <data name="ERR_TypeForwardedToMultipleAssemblies" xml:space="preserve">
-    <value>Type '{0}' is forwarded to multiple assemblies: '{1}' and '{2}'</value>
+    <value>Module '{0}' in assembly '{1}' is forwarding the type '{2}' to multiple assemblies: '{3}' and '{4}'.</value>
   </data>
 </root>
\ No newline at end of file

src/Compilers/CSharp/Portable/Symbols/AssemblySymbol.cs

@@ -370,6 +370,12 @@ internal ErrorTypeSymbol CreateCycleInTypeForwarderErrorTypeSymbol(ref MetadataT
             return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(this.Modules[0], ref emittedName, diagnosticInfo);
         }
 
+        internal ErrorTypeSymbol CreateMultipleForwardingErrorTypeSymbol(ref MetadataTypeName emittedName, ModuleSymbol forwardingModule, AssemblySymbol destination1, AssemblySymbol destination2)
+        {
+            var diagnosticInfo = new CSDiagnosticInfo(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, forwardingModule, this, emittedName.FullName, destination1, destination2);
+            return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(forwardingModule, ref emittedName, diagnosticInfo);
+        }
+
         /// <summary>
         /// Lookup declaration for predefined CorLib type in this Assembly.
         /// </summary>

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEAssemblySymbol.cs

@@ -157,13 +157,8 @@ internal override NamedTypeSymbol TryLookupForwardedMetadataTypeWithCycleDetecti
             {
                 if ((object)secondSymbol != null)
                 {
-                    var forwardingErrorInfo = new DiagnosticInfo(
-                        MessageProvider.Instance,
-                        (int)ErrorCode.ERR_TypeForwardedToMultipleAssemblies,
-                        emittedName.FullName,
-                        firstSymbol.Name,
-                        secondSymbol.Name);
-                    return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(PrimaryModule, ref emittedName, forwardingErrorInfo);
+                    // Report the main module as that is the only one checked. clr does not honor type forwarders in non-primary modules.
+                    return CreateMultipleForwardingErrorTypeSymbol(ref emittedName, this.PrimaryModule, firstSymbol, secondSymbol);
                 }
                 
                 // Don't bother to check the forwarded-to assembly if we've already seen it.

src/Compilers/CSharp/Portable/Symbols/Metadata/PE/PEModuleSymbol.cs

@@ -653,9 +653,19 @@ internal NamedTypeSymbol LookupTopLevelMetadataType(ref MetadataTypeName emitted
             string matchedName;
             (int firstIndex, int secondIndex) = this.Module.GetAssemblyRefsForForwardedType(fullName.FullName, ignoreCase: false, matchedName: out matchedName);
 
-            var firstSymbol = firstIndex < 0 ? null : GetReferencedAssemblySymbol(firstIndex);
-            var secondSymbol = secondIndex < 0 ? null : GetReferencedAssemblySymbol(secondIndex);
+            if (firstIndex < 0)
+            {
+                return (null, null);
+            }
+
+            AssemblySymbol firstSymbol = GetReferencedAssemblySymbol(firstIndex);
 
+            if (secondIndex < 0)
+            {
+                return (firstSymbol, null);
+            }
+
+            AssemblySymbol secondSymbol = GetReferencedAssemblySymbol(secondIndex);
             return (firstSymbol, secondSymbol);
         }
 
@@ -663,13 +673,9 @@ internal IEnumerable<NamedTypeSymbol> GetForwardedTypes()
         {
             foreach (KeyValuePair<string, (int FirstIndex, int SecondIndex)> forwarder in Module.GetForwardedTypes())
             {
-                if (forwarder.Value.FirstIndex < 0)
-                {
-                    continue;
-                }
-
                 var name = MetadataTypeName.FromFullName(forwarder.Key);
 
+                Debug.Assert(forwarder.Value.FirstIndex >= 0, "First index should never be negative");
                 AssemblySymbol firstSymbol = this.GetReferencedAssemblySymbol(forwarder.Value.FirstIndex);
                 Debug.Assert((object)firstSymbol != null, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()");
 
@@ -678,8 +684,7 @@ internal IEnumerable<NamedTypeSymbol> GetForwardedTypes()
                     var secondSymbol = this.GetReferencedAssemblySymbol(forwarder.Value.SecondIndex);
                     Debug.Assert((object)secondSymbol != null, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()");
 
-                    var forwardingErrorInfo = new DiagnosticInfo(MessageProvider.Instance, (int)ErrorCode.ERR_TypeForwardedToMultipleAssemblies, forwarder.Key, firstSymbol.Name, secondSymbol.Name);
-                    yield return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(this, ref name, forwardingErrorInfo);
+                    yield return ContainingAssembly.CreateMultipleForwardingErrorTypeSymbol(ref name, this, firstSymbol, secondSymbol);
                 }
                 else
                 {

src/Compilers/CSharp/Portable/Symbols/Source/SourceAssemblySymbol.cs

@@ -2589,13 +2589,7 @@ internal override NamedTypeSymbol TryLookupForwardedMetadataTypeWithCycleDetecti
                     {
                         if ((object)secondSymbol != null)
                         {
-                            var forwardingErrorInfo = new DiagnosticInfo(
-                                MessageProvider.Instance,
-                                (int)ErrorCode.ERR_TypeForwardedToMultipleAssemblies,
-                                emittedName.FullName,
-                                firstSymbol.Name,
-                                secondSymbol.Name);
-                            return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(SourceModule, ref emittedName, forwardingErrorInfo);
+                            return CreateMultipleForwardingErrorTypeSymbol(ref emittedName, peModuleSymbol, firstSymbol, secondSymbol);
                         }
 
                         // Don't bother to check the forwarded-to assembly if we've already seen it.

src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs

@@ -19508,6 +19508,7 @@ .assembly Forwarder
 {
     .ver 1:0:0:0
 }
+.module ForwarderModule.dll
 .class extern forwarder Destination.TestClass
 {
 	.assembly extern Destination1
@@ -19519,9 +19520,9 @@ .assembly Forwarder
             var compilation = CreateCompilationWithCustomILSource(userCode, forwardingIL, appendDefaultHeader: false);
 
             compilation.VerifyDiagnostics(
-                // (8,29): error CS8206: Type 'Destination.TestClass' is forwarded to multiple assemblies: 'Destination1' and 'Destination2'
+                // (8,29): error CS8206: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Destination.TestClass' to multiple assemblies: 'Destination1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' and 'Destination2, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
                 //             new Destination.TestClass();
-                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "TestClass").WithArguments("Destination.TestClass", "Destination1", "Destination2").WithLocation(8, 29),
+                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "TestClass").WithArguments("ForwarderModule.dll", "Forwarder, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "Destination.TestClass", "Destination1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "Destination2, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null").WithLocation(8, 29),
                 // (8,29): error CS0234: The type or namespace name 'TestClass' does not exist in the namespace 'Destination' (are you missing an assembly reference?)
                 //             new Destination.TestClass();
                 Diagnostic(ErrorCode.ERR_DottedTypeNameNotFoundInNS, "TestClass").WithArguments("TestClass", "Destination").WithLocation(8, 29));
@@ -19542,9 +19543,8 @@ public static void Main()
     }
 }";
             var forwardingIL = @"
-.assembly Forwarder
-{
-}
+.assembly Forwarder { }
+.module ForwarderModule.dll
 .assembly extern Destination1 { }
 .assembly extern Destination2 { }
 .assembly extern Destination3 { }
@@ -19573,9 +19573,9 @@ .assembly Forwarder
             var compilation = CreateCompilationWithCustomILSource(userCode, forwardingIL, appendDefaultHeader: false);
 
             compilation.VerifyDiagnostics(
-                // (8,29): error CS8206: Type 'Destination.TestClass' is forwarded to multiple assemblies: 'Destination1' and 'Destination2'
+                // (8,29): error CS8206: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Destination.TestClass' to multiple assemblies: 'Destination1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'Destination2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
                 //             new Destination.TestClass();
-                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "TestClass").WithArguments("Destination.TestClass", "Destination1", "Destination2").WithLocation(8, 29),
+                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "TestClass").WithArguments("ForwarderModule.dll", "Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "Destination.TestClass", "Destination1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "Destination2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null").WithLocation(8, 29),
                 // (8,29): error CS0234: The type or namespace name 'TestClass' does not exist in the namespace 'Destination' (are you missing an assembly reference?)
                 //             new Destination.TestClass();
                 Diagnostic(ErrorCode.ERR_DottedTypeNameNotFoundInNS, "TestClass").WithArguments("TestClass", "Destination").WithLocation(8, 29));
@@ -19631,6 +19631,7 @@ .assembly C
 {
 	.ver 0:0:0:0
 }
+.module CModule.dll
 .assembly extern D1 { }
 .assembly extern D2 { }
 .class extern forwarder C.ClassC
@@ -19645,9 +19646,9 @@ .assembly C
             var referenceC2 = CompileIL(codeC2, appendDefaultHeader: false);
 
             CreateCompilationWithMscorlib(codeA, references: new MetadataReference[] { referenceB, referenceC2 }, assemblyName: "A").VerifyDiagnostics(
-                // (10,13): error CS8206: Type 'C.ClassC' is forwarded to multiple assemblies: 'D1' and 'D2'
+                // (10,13): error CS8206: Module 'CModule.dll' in assembly 'C, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'C.ClassC' to multiple assemblies: 'D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
                 //             ClassB.MethodB(null);
-                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "ClassB.MethodB").WithArguments("C.ClassC", "D1", "D2").WithLocation(10, 13));
+                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies, "ClassB.MethodB").WithArguments("CModule.dll", "C, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "C.ClassC", "D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null"));
         }
 
         [Fact, WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")]
@@ -19718,9 +19719,10 @@ .assembly C
         }
 
         [Fact, WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")]
-        public void AddingReferenceToModuleWithMultipleForwardersToDifferentAssembliesShouldErrorOut()
+        public void CompilingModuleWithMultipleForwardersToDifferentAssembliesShouldErrorOut()
         {
             var ilSource = @"
+.module ForwarderModule.dll
 .assembly extern D1 { }
 .assembly extern D2 { }
 .class extern forwarder Testspace.TestType
@@ -19732,19 +19734,14 @@ public void AddingReferenceToModuleWithMultipleForwardersToDifferentAssembliesSh
 	.assembly extern D2
 }";
 
-            ImmutableArray<byte> ilBytes;
-            ImmutableArray<byte> pdbBytes;
-            EmitILToArray(ilSource, appendDefaultHeader: false, includePdb: false, assemblyBytes: out ilBytes, pdbBytes: out pdbBytes);
-
-            var ilModule = ModuleMetadata.CreateFromImage(ilBytes).GetReference();
-
-            CreateCompilationWithMscorlib(string.Empty, references: new MetadataReference[] { ilModule }).VerifyDiagnostics(
-                // error CS8206: Type 'Testspace.TestType' is forwarded to multiple assemblies: 'D1' and 'D2'
-                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies).WithArguments("Testspace.TestType", "D1", "D2").WithLocation(1, 1));
+            var ilModule = GetILModuleReference(ilSource, appendDefaultHeader: false);
+            CreateCompilationWithMscorlib(string.Empty, references: new MetadataReference[] { ilModule }, assemblyName: "Forwarder").VerifyDiagnostics(
+                // error CS8206: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Testspace.TestType' to multiple assemblies: 'D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
+                Diagnostic(ErrorCode.ERR_TypeForwardedToMultipleAssemblies).WithArguments("ForwarderModule.dll", "Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "Testspace.TestType", "D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null", "D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null").WithLocation(1, 1));
         }
 
         [Fact, WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")]
-        public void AddingReferenceToModuleWithMultipleForwardersToTheSameAssemblyShouldNotProduceMultipleForwardingErrors()
+        public void CompilingModuleWithMultipleForwardersToTheSameAssemblyShouldNotProduceMultipleForwardingErrors()
         {
             var ilSource = @"
 .assembly extern D1 { }
@@ -19757,12 +19754,7 @@ public void AddingReferenceToModuleWithMultipleForwardersToTheSameAssemblyShould
 	.assembly extern D1
 }";
 
-            ImmutableArray<byte> ilBytes;
-            ImmutableArray<byte> pdbBytes;
-            EmitILToArray(ilSource, appendDefaultHeader: false, includePdb: false, assemblyBytes: out ilBytes, pdbBytes: out pdbBytes);
-
-            var ilModule = ModuleMetadata.CreateFromImage(ilBytes).GetReference();
-
+            var ilModule = GetILModuleReference(ilSource, appendDefaultHeader: false);
             CreateCompilationWithMscorlib(string.Empty, references: new MetadataReference[] { ilModule }).VerifyDiagnostics(
                 // error CS0012: The type 'TestType' is defined in an assembly that is not referenced. You must add a reference to assembly 'D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
                 Diagnostic(ErrorCode.ERR_NoTypeDef).WithArguments("Testspace.TestType", "D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null").WithLocation(1, 1));

src/Compilers/Core/Portable/MetadataReader/PEModule.cs

@@ -2901,7 +2901,7 @@ private void EnsureForwardTypeToAssemblyMap()
                             continue;
                         }
 
-                        if (referencedAssemblyIndex >= this.ReferencedAssemblies.Length)
+                        if (referencedAssemblyIndex < 0 || referencedAssemblyIndex >= this.ReferencedAssemblies.Length)
                         {
                             continue;
                         }
@@ -2921,6 +2921,8 @@ private void EnsureForwardTypeToAssemblyMap()
 
                         if (typesToAssemblyIndexMap.TryGetValue(name, out indices))
                         {
+                            Debug.Assert(indices.FirstIndex >= 0, "Not allowed to store a negative (non-existent) index in typesToAssemblyIndexMap");
+
                             // Store it only if it was not a duplicate
                             if (indices.FirstIndex != referencedAssemblyIndex && indices.SecondIndex < 0)
                             {

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

@@ -511,9 +511,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                 Next
 
                 If forwardedType IsNot Nothing Then
-                    If Not reportedAnError And forwardedType.IsErrorType Then
+                    If Not reportedAnError AndAlso forwardedType.IsErrorType Then
                         Dim errorInfo = DirectCast(forwardedType, ErrorTypeSymbol).ErrorInfo
-                        
+
                         If errorInfo.Code = ERRID.ERR_TypeFwdCycle2 Then
                             Debug.Assert(forwardedType.ContainingAssembly IsNot Nothing, "How did we find a cycle if there is no forwarding?")
                             Binder.ReportDiagnostic(diagBag, typeSyntax, ERRID.ERR_TypeFwdCycle2, fullName, forwardedType.ContainingAssembly)

src/Compilers/VisualBasic/Portable/Symbols/AssemblySymbol.vb

@@ -301,6 +301,11 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
             Return New MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(Me.Modules(0), emittedName, diagInfo)
         End Function
 
+        Friend Function CreateMultipleForwardingErrorTypeSymbol(ByRef emittedName As MetadataTypeName, forwardingModule As ModuleSymbol, destination1 As AssemblySymbol, destination2 As AssemblySymbol) As ErrorTypeSymbol
+            Dim diagnosticInfo = New DiagnosticInfo(MessageProvider.Instance, ERRID.ERR_TypeForwardedToMultipleAssemblies, forwardingModule, Me, emittedName.FullName, destination1, destination2)
+            Return New MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(forwardingModule, emittedName, diagnosticInfo)
+        End Function
+
         ''' <summary>
         ''' Lookup declaration for predefined CorLib type in this Assembly. Only valid if this 
         ''' assembly is the Cor Library

src/Compilers/VisualBasic/Portable/Symbols/Metadata/PE/PEAssemblySymbol.vb

@@ -166,17 +166,12 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols.Metadata.PE
             Dim matchedName As String = Nothing
             Dim forwardedToAssemblies = LookupAssembliesForForwardedMetadataType(emittedName, ignoreCase, matchedName)
 
-            If DirectCast(forwardedToAssemblies.FirstSymbol, Object) IsNot Nothing
-                If DirectCast(forwardedToAssemblies.SecondSymbol, Object) IsNot Nothing
-                    Dim forwardingErrorInfo = New DiagnosticInfo(
-                        MessageProvider.Instance,
-                        ERRID.ERR_TypeForwardedToMultipleAssemblies,
-                        emittedName.FullName,
-                        forwardedToAssemblies.FirstSymbol.Name,
-                        forwardedToAssemblies.SecondSymbol.Name)
-                    Return new MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(PrimaryModule, emittedName, forwardingErrorInfo)
+            If forwardedToAssemblies.FirstSymbol IsNot Nothing Then
+                If forwardedToAssemblies.SecondSymbol IsNot Nothing Then
+                    ' Report the main module as that Is the only one checked. clr does Not honor type forwarders in non-primary modules.
+                    Return CreateMultipleForwardingErrorTypeSymbol(emittedName, PrimaryModule, forwardedToAssemblies.FirstSymbol, forwardedToAssemblies.SecondSymbol)
                 End If
-                
+
                 ' Don't bother to check the forwarded-to assembly if we've already seen it.
                 If visitedAssemblies IsNot Nothing AndAlso visitedAssemblies.Contains(forwardedToAssemblies.FirstSymbol) Then
                     Return CreateCycleInTypeForwarderErrorTypeSymbol(emittedName)

src/Compilers/VisualBasic/Portable/Symbols/Metadata/PE/PEModuleSymbol.vb

@@ -434,30 +434,34 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols.Metadata.PE
         ''' </remarks>
         Friend Function GetAssembliesForForwardedType(ByRef fullName As MetadataTypeName, ignoreCase As Boolean, <Out> ByRef matchedName As String) As (FirstSymbol As AssemblySymbol, SecondSymbol As AssemblySymbol)
             Dim indices = Me.Module.GetAssemblyRefsForForwardedType(fullName.FullName, ignoreCase, matchedName)
-            
-            Dim firstSymbol = If(indices.FirstIndex < 0, Nothing, GetReferencedAssemblySymbol(indices.FirstIndex))
-            Dim secondSymbol = If(indices.SecondIndex < 0, Nothing, GetReferencedAssemblySymbol(indices.SecondIndex))
 
-            return (firstSymbol, secondSymbol)
+            If indices.FirstIndex < 0 Then
+                Return (Nothing, Nothing)
+            End If
+
+            Dim firstSymbol = GetReferencedAssemblySymbol(indices.FirstIndex)
+
+            If indices.SecondIndex < 0 Then
+                Return (firstSymbol, Nothing)
+            End If
+
+            Dim secondSymbol = GetReferencedAssemblySymbol(indices.SecondIndex)
+            Return (firstSymbol, secondSymbol)
         End Function
 
         Friend Iterator Function GetForwardedTypes() As IEnumerable(Of NamedTypeSymbol)
             For Each forwarder As KeyValuePair(Of String, (FirstIndex As Integer, SecondIndex As Integer)) In Me.Module.GetForwardedTypes()
-                If forwarder.Value.FirstIndex < 0 Then
-                    Continue For
-                End If
-
                 Dim name = MetadataTypeName.FromFullName(forwarder.Key)
 
+                Debug.Assert(forwarder.Value.FirstIndex >= 0, "First index should never be negative")
                 Dim firstSymbol = GetReferencedAssemblySymbol(forwarder.Value.FirstIndex)
-                Debug.Assert(DirectCast(firstSymbol, Object) IsNot Nothing, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()")
+                Debug.Assert(firstSymbol IsNot Nothing, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()")
 
                 If forwarder.Value.SecondIndex >= 0 Then
                     Dim secondSymbol = GetReferencedAssemblySymbol(forwarder.Value.SecondIndex)
-                    Debug.Assert(DirectCast(secondSymbol, Object) IsNot Nothing, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()")
+                    Debug.Assert(secondSymbol IsNot Nothing, "Invalid indexes (out of bound) are discarded during reading metadata in PEModule.EnsureForwardTypeToAssemblyMap()")
 
-                    Dim forwardingErrorInfo = New DiagnosticInfo(MessageProvider.Instance, ERRID.ERR_TypeForwardedToMultipleAssemblies, forwarder.Key, firstSymbol.Name, secondSymbol.Name)
-                    Yield New MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(Me, name, forwardingErrorInfo)
+                    Yield ContainingAssembly.CreateMultipleForwardingErrorTypeSymbol(name, Me, firstSymbol, secondSymbol)
                 Else
                     Yield firstSymbol.LookupTopLevelMetadataType(name, digThroughForwardedTypes:= true)
                 End If

src/Compilers/VisualBasic/Portable/Symbols/Source/SourceAssemblySymbol.vb

@@ -1695,10 +1695,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Symbols
                     Dim peModuleSymbol = DirectCast(_modules(i), PEModuleSymbol)
                     Dim forwardedToAssemblies = peModuleSymbol.GetAssembliesForForwardedType(emittedName, ignoreCase, matchedName)
 
-                    If DirectCast(forwardedToAssemblies.FirstSymbol, Object) IsNot Nothing Then
-                        If DirectCast(forwardedToAssemblies.SecondSymbol, Object) IsNot Nothing Then
-                            Dim forwardingErrorInfo = New DiagnosticInfo(MessageProvider.Instance, ERRID.ERR_TypeForwardedToMultipleAssemblies, emittedName.FullName, forwardedToAssemblies.FirstSymbol.Name, forwardedToAssemblies.SecondSymbol.Name)
-                            Return New MissingMetadataTypeSymbol.TopLevelWithCustomErrorInfo(SourceModule, emittedName, forwardingErrorInfo)
+                    If forwardedToAssemblies.FirstSymbol IsNot Nothing Then
+                        If forwardedToAssemblies.SecondSymbol IsNot Nothing Then
+                            Return CreateMultipleForwardingErrorTypeSymbol(emittedName, peModuleSymbol, forwardedToAssemblies.FirstSymbol, forwardedToAssemblies.SecondSymbol)
                         End If
 
                         ' Don't bother to check the forwarded-to assembly if we've already seen it.

src/Compilers/VisualBasic/Portable/VBResources.Designer.vb

@@ -10686,7 +10686,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         End Property
         
         '''<summary>
-        '''  Looks up a localized string similar to Type &apos;{0}&apos; is forwarded to multiple assemblies: &apos;{1}&apos; and &apos;{2}&apos;.
+        '''  Looks up a localized string similar to Module &apos;{0}&apos; in assembly &apos;{1}&apos; is forwarding the type &apos;{2}&apos; to multiple assemblies: &apos;{3}&apos; and &apos;{4}&apos;..
         '''</summary>
         Friend ReadOnly Property ERR_TypeForwardedToMultipleAssemblies() As String
             Get

src/Compilers/VisualBasic/Portable/VBResources.resx

@@ -5462,6 +5462,6 @@
     <value>Invalid assembly name: {0}</value>
   </data>
   <data name="ERR_TypeForwardedToMultipleAssemblies" xml:space="preserve">
-    <value>Type '{0}' is forwarded to multiple assemblies: '{1}' and '{2}'</value>
+    <value>Module '{0}' in assembly '{1}' is forwarding the type '{2}' to multiple assemblies: '{3}' and '{4}'.</value>
   </data>
-</root>
+</root>
\ No newline at end of file

src/Compilers/VisualBasic/Test/Symbol/SymbolsTests/SymbolErrorTests.vb

@@ -23844,6 +23844,7 @@ End Namespace
 {
     .ver 1:0:0:0
 }
+.module ForwarderModule.dll
 .class extern forwarder Destination.TestClass
 {
 	.assembly extern Destination1
@@ -23853,12 +23854,12 @@ End Namespace
 	.assembly extern Destination2
 }"
             Dim compilation = CreateCompilationWithCustomILSource(userCode, forwardingIL, appendDefaultHeader:= False)
-            
+
             CompilationUtils.AssertTheseDiagnostics(compilation, <errors><![CDATA[
 BC30002: Type 'Destination.TestClass' is not defined.
             Dim obj = New Destination.TestClass()
                           ~~~~~~~~~~~~~~~~~~~~~
-BC37208: Type 'Destination.TestClass' is forwarded to multiple assemblies: 'Destination1' and 'Destination2'
+BC37208: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Destination.TestClass' to multiple assemblies: 'Destination1, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' and 'Destination2, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
             Dim obj = New Destination.TestClass()
                           ~~~~~~~~~~~~~~~~~~~~~
  ]]></errors>)
@@ -23885,6 +23886,7 @@ End Namespace
 .assembly Forwarder
 {
 }
+.module ForwarderModule.dll
 .assembly extern Destination1 { }
 .assembly extern Destination2 { }
 .assembly extern Destination3 { }
@@ -23912,12 +23914,12 @@ End Namespace
 }"
 
             Dim compilation = CreateCompilationWithCustomILSource(userCode, forwardingIL, appendDefaultHeader:= False)
-            
+
             CompilationUtils.AssertTheseDiagnostics(compilation, <errors><![CDATA[
 BC30002: Type 'Destination.TestClass' is not defined.
             Dim obj = New Destination.TestClass()
                           ~~~~~~~~~~~~~~~~~~~~~
-BC37208: Type 'Destination.TestClass' is forwarded to multiple assemblies: 'Destination1' and 'Destination2'
+BC37208: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Destination.TestClass' to multiple assemblies: 'Destination1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'Destination2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
             Dim obj = New Destination.TestClass()
                           ~~~~~~~~~~~~~~~~~~~~~
  ]]></errors>)
@@ -23984,6 +23986,7 @@ End Namespace"
 {
 	.ver 0:0:0:0
 }
+.module CModule.dll
 .assembly extern D1 { }
 .assembly extern D2 { }
 .class extern forwarder C.ClassC
@@ -24004,7 +24007,7 @@ End Namespace"
                 assemblyName:= "A")
 
             CompilationUtils.AssertTheseDiagnostics(compilation, <errors><![CDATA[
-BC37208: Type 'C.ClassC' is forwarded to multiple assemblies: 'D1' and 'D2'
+BC37208: Module 'CModule.dll' in assembly 'C, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'C.ClassC' to multiple assemblies: 'D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
             ClassB.MethodB(Nothing)
             ~~~~~~~~~~~~~~~~~~~~~~~
  ]]></errors>)
@@ -24092,8 +24095,9 @@ BC30652: Reference required to assembly 'D, Version=0.0.0.0, Culture=neutral, Pu
 
         <Fact>
         <WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")>
-        Public Sub AddingReferenceToModuleWithMultipleForwardersToDifferentAssembliesShouldErrorOut()
+        Public Sub CompilingModuleWithMultipleForwardersToDifferentAssembliesShouldErrorOut()
             Dim ilSource = "
+.module ForwarderModule.dll
 .assembly extern D1 { }
 .assembly extern D2 { }
 .class extern forwarder Testspace.TestType
@@ -24105,22 +24109,21 @@ BC30652: Reference required to assembly 'D, Version=0.0.0.0, Culture=neutral, Pu
 	.assembly extern D2
 }"
 
-            Dim ilBytes As ImmutableArray(Of Byte) = Nothing
-            Dim pdbBytes As ImmutableArray(Of Byte) = Nothing
-            EmitILToArray(ilSource, appendDefaultHeader:=False, includePdb:=False, assemblyBytes:=ilBytes, pdbBytes:=pdbBytes)
-
-            Dim ilModule = ModuleMetadata.CreateFromImage(ilBytes).GetReference()
-
-            Dim compilation = CreateCompilationWithMscorlib(String.Empty, references:={ilModule}, options:=New VisualBasicCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
+            Dim ilModule = GetILModuleReference(ilSource, appendDefaultHeader:=False)
+            Dim compilation = CreateCompilationWithMscorlib(
+                source:=String.Empty,
+                references:={ilModule},
+                options:=New VisualBasicCompilationOptions(OutputKind.DynamicallyLinkedLibrary),
+                assemblyName:="Forwarder")
 
             CompilationUtils.AssertTheseDiagnostics(compilation, <errors><![CDATA[
-BC37208: Type 'Testspace.TestType' is forwarded to multiple assemblies: 'D1' and 'D2'
+BC37208: Module 'ForwarderModule.dll' in assembly 'Forwarder, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' is forwarding the type 'Testspace.TestType' to multiple assemblies: 'D1, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' and 'D2, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'.
 ]]></errors>)
         End Sub
 
         <Fact>
-                                      <WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")>
-        Public Sub AddingReferenceToModuleWithMultipleForwardersToTheSameAssemblyShouldNotProduceMultipleForwardingErrors()
+        <WorkItem(16484, "https://github.com/dotnet/roslyn/issues/16484")>
+        Public Sub CompilingModuleWithMultipleForwardersToTheSameAssemblyShouldNotProduceMultipleForwardingErrors()
             Dim ilSource = "
 .assembly extern D1 { }
 .class extern forwarder Testspace.TestType
@@ -24132,12 +24135,7 @@ BC37208: Type 'Testspace.TestType' is forwarded to multiple assemblies: 'D1' and
 	.assembly extern D1
 }"
 
-            Dim ilBytes As ImmutableArray(Of Byte) = Nothing
-            Dim pdbBytes As ImmutableArray(Of Byte) = Nothing
-            EmitILToArray(ilSource, appendDefaultHeader:=False, includePdb:=False, assemblyBytes:=ilBytes, pdbBytes:=pdbBytes)
-
-            Dim ilModule = ModuleMetadata.CreateFromImage(ilBytes).GetReference()
-
+            Dim ilModule = GetILModuleReference(ilSource, appendDefaultHeader:=False)
             Dim compilation = CreateCompilationWithMscorlib(String.Empty, references:={ilModule}, options:=New VisualBasicCompilationOptions(OutputKind.DynamicallyLinkedLibrary))
 
             CompilationUtils.AssertTheseDiagnostics(compilation, <errors><![CDATA[

src/Test/Utilities/Portable/CommonTestBase.cs

@@ -267,6 +267,14 @@ internal static MetadataReference CompileIL(string ilSource, bool appendDefaultH
             return AssemblyMetadata.CreateFromImage(assemblyBytes).GetReference(embedInteropTypes: embedInteropTypes);
         }
 
+        internal static MetadataReference GetILModuleReference(string ilSource, bool appendDefaultHeader = true)
+        {
+            ImmutableArray<byte> assemblyBytes;
+            ImmutableArray<byte> pdbBytes;
+            EmitILToArray(ilSource, appendDefaultHeader, includePdb: false, assemblyBytes: out assemblyBytes, pdbBytes: out pdbBytes);
+            return ModuleMetadata.CreateFromImage(assemblyBytes).GetReference();
+        }
+
         #endregion
 
         #region Compilation Creation Helpers