Improve diagnostic for ambiguous predefined type

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -1601,7 +1601,7 @@ If such a class is used as a base class and if the deriving class defines a dest
   <data name="ERR_PredefinedValueTupleTypeNotFound" xml:space="preserve">
     <value>Predefined type '{0}' is not defined or imported</value>
   </data>
-  <data name="ERR_PredefinedValueTupleTypeAmbiguous3" xml:space="preserve">
+  <data name="ERR_PredefinedTypeAmbiguous" xml:space="preserve">
     <value>Predefined type '{0}' is declared in multiple referenced assemblies: '{1}' and '{2}'</value>
   </data>
   <data name="ERR_StructWithBaseConstructorCall" xml:space="preserve">

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1544,7 +1544,7 @@ internal enum ErrorCode
         ERR_OutAttrOnInParam = 8355,
         #endregion diagnostics introduced for `ref readonly`, `ref conditional` and `ref-like` features in C# 7.2
 
-        ERR_PredefinedValueTupleTypeAmbiguous3 = 8356,
+        ERR_PredefinedTypeAmbiguous = 8356,
         ERR_InvalidVersionFormatDeterministic = 8357,
         ERR_AttributeCtorInParameter = 8358,
 

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -1868,7 +1868,7 @@ or ErrorCode.ERR_EscapeVariable
                 or ErrorCode.ERR_EscapeStackAlloc
                 or ErrorCode.ERR_RefReturnThis
                 or ErrorCode.ERR_OutAttrOnInParam
-                or ErrorCode.ERR_PredefinedValueTupleTypeAmbiguous3
+                or ErrorCode.ERR_PredefinedTypeAmbiguous
                 or ErrorCode.ERR_InvalidVersionFormatDeterministic
                 or ErrorCode.ERR_AttributeCtorInParameter
                 or ErrorCode.WRN_FilterIsConstantFalse

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

@@ -154,25 +154,27 @@ internal NamedTypeSymbol GetWellKnownType(WellKnownType type)
                 if (result is null)
                 {
                     MetadataTypeName emittedName = MetadataTypeName.FromFullName(mdName, useCLSCompliantNameArityEncoding: true);
-                    if (type.IsValueTupleType())
+
+                    CSDiagnosticInfo? errorInfo;
+                    if (conflicts is ({ } conflict1, { } conflict2))
                     {
-                        CSDiagnosticInfo errorInfo;
-                        if (conflicts.Item1 is null)
+                        errorInfo = new CSDiagnosticInfo(ErrorCode.ERR_PredefinedTypeAmbiguous, emittedName.FullName, conflict1, conflict2);
+                    }
+                    else
+                    {
+                        Debug.Assert(conflicts is (null, null));
+
+                        if (type.IsValueTupleType())
                         {
-                            Debug.Assert(conflicts.Item2 is null);
                             errorInfo = new CSDiagnosticInfo(ErrorCode.ERR_PredefinedValueTupleTypeNotFound, emittedName.FullName);
                         }
                         else
                         {
-                            errorInfo = new CSDiagnosticInfo(ErrorCode.ERR_PredefinedValueTupleTypeAmbiguous3, emittedName.FullName, conflicts.Item1, conflicts.Item2);
+                            errorInfo = null;
                         }
-
-                        result = new MissingMetadataTypeSymbol.TopLevel(this.Assembly.Modules[0], ref emittedName, type, errorInfo);
-                    }
-                    else
-                    {
-                        result = new MissingMetadataTypeSymbol.TopLevel(this.Assembly.Modules[0], ref emittedName, type);
                     }
+
+                    result = new MissingMetadataTypeSymbol.TopLevel(this.Assembly.Modules[0], ref emittedName, type, errorInfo);
                 }
 
                 if (Interlocked.CompareExchange(ref _lazyWellKnownTypes[index], result, null) is object)