[SE-0338] Perform Sendable checking when exiting an actor via conformance.

When a requirement is actor-isolated and asynchronous but its witness is
non-isolated, perform Sendable checking on the requirement's type.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -2955,6 +2955,10 @@ bool ConformanceChecker::checkActorIsolation(
     requirementIsolation = requirementIsolation.subst(subs);
   }
 
+  SourceLoc loc = witness->getLoc();
+  if (loc.isInvalid())
+    loc = Conformance->getLoc();
+
   auto refResult = ActorReferenceResult::forReference(
       getConcreteWitness(), witness->getLoc(), DC, None, None,
       None, requirementIsolation);
@@ -2972,7 +2976,8 @@ bool ConformanceChecker::checkActorIsolation(
     return false;
 
   case ActorReferenceResult::ExitsActorToNonisolated:
-    // FIXME: SE-0338 would diagnose this.
+    diagnoseNonSendableTypesInReference(
+        getConcreteWitness(), DC, loc, SendableCheckReason::Conformance);
     return false;
 
   case ActorReferenceResult::EntersActor:
@@ -3016,10 +3021,6 @@ bool ConformanceChecker::checkActorIsolation(
 
   // If we aren't missing anything, do a Sendable check and move on.
   if (!missingOptions) {
-    SourceLoc loc = witness->getLoc();
-    if (loc.isInvalid())
-      loc = Conformance->getLoc();
-
     // FIXME: Disable Sendable checking when the witness is an initializer
     // that is explicitly marked nonisolated.
     if (isa<ConstructorDecl>(witness) &&

test/Concurrency/sendable_conformance_checking.swift

@@ -2,7 +2,7 @@
 // REQUIRES: concurrency
 
 @available(SwiftStdlib 5.1, *)
-class NotSendable { // expected-note 8{{class 'NotSendable' does not conform to the 'Sendable' protocol}}
+class NotSendable { // expected-note 9{{class 'NotSendable' does not conform to the 'Sendable' protocol}}
 }
 
 @available(SwiftStdlib 5.1, *)
@@ -13,13 +13,16 @@ extension NotSendable: Sendable { }
 protocol IsolatedWithNotSendableRequirements: Actor {
   func f() -> NotSendable
   var prop: NotSendable { get }
+
+  func fAsync() async -> NotSendable
 }
 
 // Okay, everything is isolated the same way
 @available(SwiftStdlib 5.1, *)
 actor A1: IsolatedWithNotSendableRequirements {
   func f() -> NotSendable { NotSendable() }
   var prop: NotSendable { NotSendable() }
+  func fAsync() async -> NotSendable { NotSendable() }
 }
 
 // Okay, sendable checking occurs when calling through the protocol
@@ -28,6 +31,9 @@ actor A1: IsolatedWithNotSendableRequirements {
 actor A2: IsolatedWithNotSendableRequirements {
   nonisolated func f() -> NotSendable { NotSendable() }
   nonisolated var prop: NotSendable { NotSendable() }
+
+  nonisolated func fAsync() async -> NotSendable { NotSendable() }
+  // expected-warning@-1{{non-sendable type 'NotSendable' returned by nonisolated instance method 'fAsync()' satisfying protocol requirement cannot cross actor boundary}}
 }
 
 @available(SwiftStdlib 5.1, *)