clarify assertion comment: some actor members can be @objc.

An actor can have @objc members, but not if those members are "actor-isolated",
i.e., not accessible from outside the actor according to the fundamental design
of actors. For example, a sync function is considered actor-isolated since it
needs special protection, so it cannot be @objc.

But, we now have special capabilities to treat sync actor functions as
implicitly async at use-sites outside of the actor. Does this mean that we can
now allow sync actor functions to be @objc? No. Because the implicitly-async
functionality does not extend to the world of ObjC: it simply would not be
feasible to implement it there.

Thus, I've added some extra regression-test coverage to handle these cases,
and clarified the assertion here to not confuse others.

Author: kavon

lib/SILGen/SILGenExpr.cpp

@@ -2198,7 +2198,7 @@ RValue RValueEmitter::visitMemberRefExpr(MemberRefExpr *e,
 
 RValue RValueEmitter::visitDynamicMemberRefExpr(DynamicMemberRefExpr *E,
                                                 SGFContext C) {
-  assert(!E->isImplicitlyAsync() && "actors do not have @objc members");
+  assert(!E->isImplicitlyAsync() && "an actor-isolated @objc member?");
   return SGF.emitDynamicMemberRefExpr(E, C);
 }
 
@@ -2220,7 +2220,7 @@ RValue RValueEmitter::visitSubscriptExpr(SubscriptExpr *E, SGFContext C) {
 
 RValue RValueEmitter::visitDynamicSubscriptExpr(
                                       DynamicSubscriptExpr *E, SGFContext C) {
-  assert(!E->isImplicitlyAsync() && "actors do not have @objc members");
+  assert(!E->isImplicitlyAsync() && "an actor-isolated @objc member?");
   return SGF.emitDynamicSubscriptExpr(E, C);
 }
 

test/Concurrency/actor_isolation_objc.swift

@@ -16,17 +16,31 @@ actor A {
   func keypaths() {
     _ = #keyPath(A.x) // expected-error{{argument of '#keyPath' refers to non-'@objc' property 'x'}}
     _ = #keyPath(A.y) // expected-error{{argument of '#keyPath' refers to non-'@objc' property 'y'}}
+    _ = #keyPath(A.computed) // expected-error{{argument of '#keyPath' refers to non-'@objc' property 'computed'}}
     _ = #keyPath(A.z)
   }
 
   var x: Int = 0 // expected-note{{add '@objc' to expose this property to Objective-C}}
+
   @objc var y: Int = 0 // expected-note{{add '@objc' to expose this property to Objective-C}}
   // expected-error@-1{{actor-isolated property 'y' cannot be @objc}}
+
   @objc @actorIndependent(unsafe) var z: Int = 0
 
+  // expected-note@+1 {{add '@objc' to expose this property to Objective-C}}
+  @objc var computed : Int { // expected-error{{actor-isolated property 'computed' cannot be @objc}}
+    get { 120 }
+  }
+
   func f() { } // expected-note{{add '@objc' to expose this instance method to Objective-C}}
   func g() { } // expected-note{{add '@objc' to expose this instance method to Objective-C}}
   @objc func h() async { }
+
+  @objc @asyncHandler func i() {}
+}
+
+func outside(a : A) async {
+  await a.i() // expected-warning {{no 'async' operations occur within 'await' expression}}
 }