[mcs] Better recovery from failed async return type inference. Fixes #25165

Author: marek-safar

mcs/errors/cs0126-3.cs

@@ -0,0 +1,26 @@
+// CS0126: An object of a type convertible to `int' is required for the return statement
+// Line: 15
+
+using System.Threading.Tasks;
+
+class MainClass
+{
+    public static void Main ()
+    {
+        Task<C> v = null;
+
+        Task.Run (async () => {
+            await Task.Yield ();
+            if (v == null) {
+                return;
+            }
+
+            return 1;
+        });
+    }
+}
+
+public class C
+{
+    string Id { get; set; }
+}

mcs/mcs/statement.cs

@@ -1135,7 +1135,7 @@ protected override bool DoResolve (BlockContext ec)
 			var block_return_type = ec.ReturnType;
 
 			if (expr == null) {
-				if (block_return_type.Kind == MemberKind.Void)
+				if (block_return_type.Kind == MemberKind.Void || block_return_type == InternalType.ErrorType)
 					return true;
 
 				//