[Concurrency] Ban asynchronous methods returning 'Self' from being @objc

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4129,6 +4129,8 @@ NOTE(objc_ambiguous_async_convention_candidate,none,
 ERROR(satisfy_async_objc,none,
       "satisfying an asychronous @objc %select{method|initializer}0 with "
       "a synchronous %select{method|initializer}0 is not supported", (bool))
+ERROR(async_objc_dynamic_self,none,
+      "asynchronous method returning 'Self' cannot be '@objc'", ())
 
 //------------------------------------------------------------------------------
 // MARK: Type Check Types

lib/Sema/TypeCheckDeclObjC.cpp

@@ -630,6 +630,18 @@ bool swift::isRepresentableInObjC(
       return false;
     }
 
+    // The completion handler transformation cannot properly represent a
+    // dynamic 'Self' type, so disallow @objc for such methods.
+    if (FD->hasDynamicSelfResult()) {
+      if (Diagnose) {
+        AFD->diagnose(diag::async_objc_dynamic_self)
+            .highlight(AFD->getAsyncLoc());
+        describeObjCReason(AFD, Reason);
+      }
+
+      return false;
+    }
+
     // The completion handler parameter always goes at the end.
     unsigned completionHandlerParamIndex = AFD->getParameters()->size();
 

test/attr/attr_objc_async.swift

@@ -17,4 +17,7 @@ class Concurrency {
 
   @objc func takeAnAsync(_ fn: () async -> Int) { } // expected-error{{method cannot be marked @objc because the type of the parameter cannot be represented in Objective-C}}
   // expected-note@-1{{'async' function types cannot be represented in Objective-C}}
+
+  @objc class func createAsynchronously() async -> Self? { nil }
+  // expected-error@-1{{asynchronous method returning 'Self' cannot be '@objc'}}
 }