Use specific error message for wrong arity in async builder

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -3743,6 +3743,9 @@ Give the compiler some way to differentiate the methods. For example, you can gi
   <data name="ERR_BadAsyncReturn" xml:space="preserve">
     <value>The return type of an async method must be void, Task, Task&lt;T&gt;, a task-like type, IAsyncEnumerable&lt;T&gt;, or IAsyncEnumerator&lt;T&gt;</value>
   </data>
+  <data name="ERR_WrongArityAsyncReturn" xml:space="preserve">
+    <value>A generic task-like return type was expected, but the type '{0}' found in 'AsyncMethodBuilder' attribute was not suitable. It must be an unbound generic type of arity one, and its containing type (if any) must be non-generic.</value>
+  </data>
   <data name="ERR_CantReturnVoid" xml:space="preserve">
     <value>Cannot return an expression of type 'void'</value>
   </data>

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

@@ -1962,6 +1962,7 @@ internal enum ErrorCode
         ERR_SimpleProgramIsEmpty = 8937,
         ERR_LineSpanDirectiveInvalidValue = 8938,
         ERR_LineSpanDirectiveEndLessThanStart = 8939,
+        ERR_WrongArityAsyncReturn = 8940,
 
         #endregion
 

src/Compilers/CSharp/Portable/Lowering/AsyncRewriter/AsyncMethodBuilderMemberCollection.cs

@@ -345,10 +345,20 @@ private static NamedTypeSymbol ValidateBuilderType(SyntheticBoundNodeFactory F,
                  !builderType.IsVoidType() &&
                  (forMethodLevelBuilder || builderType.DeclaredAccessibility == desiredAccessibility))
             {
-                bool isArityOk = isGeneric
-                                 ? builderType.IsUnboundGenericType && builderType.ContainingType?.IsGenericType != true && builderType.Arity == 1
-                                 : !builderType.IsGenericType;
-                if (isArityOk)
+                if (isGeneric)
+                {
+                    if (builderType.IsUnboundGenericType && builderType.ContainingType?.IsGenericType != true && builderType.Arity == 1)
+                    {
+                        return builderType;
+                    }
+                    else
+                    {
+                        F.Diagnostics.Add(ErrorCode.ERR_WrongArityAsyncReturn, F.Syntax.Location, builderType);
+                        return null;
+                    }
+                }
+
+                if (!builderType.IsGenericType)
                 {
                     return builderType;
                 }