A "defer" block is global-actor-isolated if its context is.

Fixes rdar://80799233.

(cherry picked from commit 5dc4fcf)

Author: DougGregor

lib/Sema/TypeCheckConcurrency.cpp

@@ -3126,6 +3126,26 @@ ActorIsolation ActorIsolationRequest::evaluate(
     return inferred;
   };
 
+  // If this is a "defer" function body, inherit the global actor isolation
+  // from its context.
+  if (auto func = dyn_cast<FuncDecl>(value)) {
+    if (func->isDeferBody()) {
+      switch (auto enclosingIsolation =
+                  getActorIsolationOfContext(func->getDeclContext())) {
+      case ActorIsolation::ActorInstance:
+      case ActorIsolation::DistributedActorInstance:
+      case ActorIsolation::Independent:
+      case ActorIsolation::Unspecified:
+        // Do nothing.
+        break;
+
+      case ActorIsolation::GlobalActor:
+      case ActorIsolation::GlobalActorUnsafe:
+        return inferredIsolation(enclosingIsolation);
+      }
+    }
+  }
+
   // If the declaration overrides another declaration, it must have the same
   // actor isolation.
   if (auto overriddenValue = value->getOverriddenDecl()) {

test/Concurrency/global_actor_inference.swift

@@ -552,3 +552,17 @@ func acceptAsyncSendableClosureInheriting<T>(@_inheritActorContext _: @Sendable
     await onlyOnMainActor() // expected-warning{{no 'async' operations occur within 'await' expression}}
   }
 }
+
+
+// defer bodies inherit global actor-ness
+@MainActor
+var statefulThingy: Bool = false
+
+@MainActor
+func useFooInADefer() -> String {
+  defer {
+    statefulThingy = true
+  }
+
+  return "hello"
+}