Open existential actors when retrieving their executors.

Fixes rdar://91448324.

Author: DougGregor

lib/SILGen/SILGenProlog.cpp

@@ -768,6 +768,15 @@ SILValue SILGenFunction::emitLoadActorExecutor(SILLocation loc,
   else
     actorV = actor.borrow(*this, loc).getValue();
 
+  // Open an existential actor type.
+  CanType actorType = actor.getType().getASTType();
+  if (actorType->isExistentialType()) {
+    actorType = OpenedArchetypeType::get(
+        actorType, F.getGenericSignature())->getCanonicalType();
+    SILType loweredActorType = getLoweredType(actorType);
+    actorV = B.createOpenExistentialRef(loc, actorV, loweredActorType);
+  }
+
   // For now, we just want to emit a hop_to_executor directly to the
   // actor; LowerHopToActor will add the emission logic necessary later.
   return actorV;

test/SILGen/isolated_parameters.swift

@@ -24,3 +24,14 @@ public func takeClosureWithIsolatedParam(body: (isolated A) async -> Void) { }
 public func testClosureWithIsolatedParam() {
   takeClosureWithIsolatedParam { _ in }
 }
+
+// CHECK-LABEL: sil{{.*}} [ossa] @$s4test0A19IsolatedExistentialyyScA_pYiYaF
+// CHECK: bb0([[ACTOR:%.*]] : @guaranteed $Actor)
+@available(SwiftStdlib 5.1, *)
+public func testIsolatedExistential(_ a: isolated Actor) async {
+  // CHECK: [[ACTOR_COPY:%.*]] = copy_value [[ACTOR]] : $Actor
+  // CHECK: [[ACTOR_BORROW:%.*]] = begin_borrow [[ACTOR_COPY]] : $Actor
+  // CHECK: [[ACTOR_OPENED:%.*]] = open_existential_ref [[ACTOR_BORROW]] : $Actor to $@opened("{{.*}}") Actor
+  // CHECK: hop_to_executor [[ACTOR_OPENED]] : $@opened("{{.*}}") Actor
+  // CHECK: return
+}