Block manual application of MethodImplAttribute.Async to methods

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -8228,4 +8228,8 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_ExtensionBlockCollision" xml:space="preserve">
     <value>This extension block collides with another extension block. They result in conflicting content-based type names in metadata, so must be in separate enclosing static classes.</value>
   </data>
+  <data name="ERR_MethodImplAttributeAsyncCannotBeUsed" xml:space="preserve">
+    <value>'MethodImplAttribute.Async' cannot be manually applied to methods. Mark the method 'async'.</value>
+    <remarks>'MethodImplAttribute.Async' and 'async' are not localizable.</remarks>
+  </data>
 </root>

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

@@ -2424,6 +2424,7 @@ internal enum ErrorCode
         ERR_FeatureNotAvailableInVersion14 = 9327,
         ERR_UnsupportedFeatureInRuntimeAsync = 9328,
         ERR_ExtensionBlockCollision = 9329,
+        ERR_MethodImplAttributeAsyncCannotBeUsed = 9330,
 
         // Note: you will need to do the following after adding errors:
         //  1) Update ErrorFacts.IsBuildOnlyDiagnostic (src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs)

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

@@ -2535,6 +2535,7 @@ or ErrorCode.ERR_InterpolatedStringHandlerArgumentDisallowed
                 or ErrorCode.ERR_MemberNameSameAsExtendedType
                 or ErrorCode.ERR_FeatureNotAvailableInVersion14
                 or ErrorCode.ERR_ExtensionBlockCollision
+                or ErrorCode.ERR_MethodImplAttributeAsyncCannotBeUsed
                     => false,
             };
 #pragma warning restore CS8524 // The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value.

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

@@ -234,6 +234,7 @@ public override void ReportDuplicateMetadataReferenceWeak(DiagnosticBag diagnost
 
         // pseudo-custom attributes:
         public override int ERR_PermissionSetAttributeFileReadError => (int)ErrorCode.ERR_PermissionSetAttributeFileReadError;
+        public override int ERR_MethodImplAttributeAsyncCannotBeUsed => (int)ErrorCode.ERR_MethodImplAttributeAsyncCannotBeUsed;
 
         // PDB Writer:
         public override int ERR_EncodinglessSyntaxTree => (int)ErrorCode.ERR_EncodinglessSyntaxTree;

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

@@ -535,7 +535,7 @@ private void DecodeWellKnownAttributeAppliedToMethod(ref DecodeWellKnownAttribut
             }
             else if (attribute.IsTargetAttribute(AttributeDescription.MethodImplAttribute))
             {
-                AttributeData.DecodeMethodImplAttribute<MethodWellKnownAttributeData, AttributeSyntax, CSharpAttributeData, AttributeLocation>(ref arguments, MessageProvider.Instance);
+                AttributeData.DecodeMethodImplAttribute<MethodWellKnownAttributeData, AttributeSyntax, CSharpAttributeData, AttributeLocation>(ref arguments, MessageProvider.Instance, this.ContainingType);
             }
             else if (attribute.IsTargetAttribute(AttributeDescription.DllImportAttribute))
             {

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

@@ -2520,6 +2520,8 @@ internal ITypeSymbol GetITypeSymbol(CodeAnalysis.NullableAnnotation nullableAnno
 
         SpecialType ITypeSymbolInternal.SpecialType => this.SpecialType;
 
+        ExtendedSpecialType ITypeSymbolInternal.ExtendedSpecialType => this.ExtendedSpecialType;
+
         bool ITypeSymbolInternal.IsReferenceType => this.IsReferenceType;
 
         bool ITypeSymbolInternal.IsValueType => this.IsValueType;