only warn for isolation mismatch in @preconcurrency context

Much like how we do for an isolation mismatch for a call within
a closure that is `@preconcurrency`, references should also only
warn about the isolation issue too.

The reason why there is a mismatch is that the completion-handlers
are considered `@Sendable`, which disables the isolation inheriting
behavior of other, non-Sendable closures.

resolves rdar://96830159

(cherry picked from commit e26c165)

Author: kavon

lib/Sema/TypeCheckConcurrency.cpp

@@ -2859,13 +2859,18 @@ namespace {
           }
         }
 
+        // Does the reference originate from a @preconcurrency context?
+        bool preconcurrencyContext =
+          result.options.contains(ActorReferenceResult::Flags::Preconcurrency);
+
         ctx.Diags.diagnose(
             loc, diag::actor_isolated_non_self_reference,
             decl->getDescriptiveKind(),
             decl->getName(),
             useKind,
             refKind + 1, refGlobalActor,
-            result.isolation);
+            result.isolation)
+          .warnUntilSwiftVersionIf(preconcurrencyContext, 6);
 
         noteIsolatedActorMember(decl, context);
 
@@ -5119,6 +5124,10 @@ ActorReferenceResult ActorReferenceResult::forReference(
   // to perform.
   Options options = None;
 
+  // Note if the reference originates from a @preconcurrency-isolated context.
+  if (contextIsolation.preconcurrency())
+    options |= Flags::Preconcurrency;
+
   // If the declaration isn't asynchronous, promote to async.
   if (!isAsyncDecl(declRef))
     options |= Flags::AsyncPromotion;

lib/Sema/TypeCheckConcurrency.h

@@ -210,6 +210,9 @@ struct ActorReferenceResult {
     /// The declaration is being accessed from outside the actor and
     /// potentially from a different node, so it must be marked 'distributed'.
     Distributed = 1 << 2,
+
+    /// The declaration is being accessed from a @preconcurrency context.
+    Preconcurrency = 1 << 3,
   };
 
   using Options = OptionSet<Flags>;

test/ClangImporter/objc_async.swift

@@ -381,3 +381,17 @@ public struct SomeWrapper<T: AuditedNonSendable> {
 }
 
 extension SomeWrapper: Sendable where T: Sendable {}
+
+
+// rdar://96830159
+@MainActor class SendableCompletionHandler {
+  var isolatedThing: [String] = []
+  // expected-note@-1 {{property declared here}}
+
+  func makeCall(slowServer: SlowServer) {
+    slowServer.doSomethingSlow("churn butter") { (_ : Int) in
+      let _ = self.isolatedThing
+      // expected-warning@-1 {{main actor-isolated property 'isolatedThing' can not be referenced from a Sendable closure; this is an error in Swift 6}}
+    }
+  }
+}