[Concurrency] Only apply default main actor to local and nested

decls that are in a main actor isolated context.

This prevents `@MainActor` from being inferred in a context where
it cannot be used, while still allowing main actor code to be used
in local contexts that are also main actor isolated.

(cherry picked from commit 858e145)

Author: hborla

lib/Sema/TypeCheckConcurrency.cpp

@@ -6125,28 +6125,33 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
     auto globalActorHelper = [&](Type globalActor)
         -> std::optional<std::tuple<InferredActorIsolation, ValueDecl *,
                                     std::optional<ActorIsolation>>> {
-      // Default global actor isolation does not apply to any declarations
-      // within actors and distributed actors, nor does it apply in a
-      // nonisolated type.
+      // Default main actor only applies to top-level declarations and
+      // in contexts that are also main actor isolated. It does not apply
+      // in {distributed} actor-isolated contexts nor in nonisolated
+      // contexts.
+
+      // Local declarations must check the isolation of their
+      // decl context.
+      if (value->getDeclContext()->isLocalContext()) {
+        auto contextIsolation =
+            getActorIsolationOfContext(value->getDeclContext());
+        if (!contextIsolation.isMainActor())
+          return {};
+      }
+
+      // Members and nested types must check the isolation of the enclosing
+      // nominal type.
       auto *dc = value->getInnermostDeclContext();
       while (dc) {
         if (auto *nominal = dc->getSelfNominalTypeDecl()) {
           if (nominal->isAnyActor())
             return {};
 
           if (dc != dyn_cast<DeclContext>(value)) {
-            switch (getActorIsolation(nominal)) {
-            case ActorIsolation::Unspecified:
-            case ActorIsolation::ActorInstance:
-            case ActorIsolation::Nonisolated:
-            case ActorIsolation::NonisolatedUnsafe:
-            case ActorIsolation::Erased:
-            case ActorIsolation::CallerIsolationInheriting:
-              return {};
-
-            case ActorIsolation::GlobalActor:
+            if (getActorIsolation(nominal).isMainActor())
               break;
-            }
+
+            return {};
           }
         }
         dc = dc->getParent();

test/Concurrency/assume_mainactor.swift

@@ -226,3 +226,46 @@ actor MyActor2 {
   // NonIsolated and not if print was inferred to be main actor.
   print("123")
 }
+
+nonisolated func localDeclIsolation() async {
+  struct Local {
+    static func f() {}
+  }
+
+  Local.f()
+
+  await withTaskGroup { group in
+    group.addTask { }
+
+    await group.next()
+  }
+}
+
+@CustomActor
+class CustomActorIsolated {
+  struct Nested {
+    // CHECK: // static CustomActorIsolated.Nested.f()
+    // CHECK-NEXT: // Isolation: unspecified
+    static func f() {}
+  }
+
+  // CHECK: // CustomActorIsolated.customIsolated()
+  // CHECK-NEXT: // Isolation: global_actor. type: CustomActor
+  func customIsolated() {
+    Nested.f()
+  }
+}
+
+var global = 0
+
+func onMain() {
+  struct Nested {
+    // CHECK: // static useGlobal() in Nested #1 in onMain()
+    // CHECK-NEXT: // Isolation: global_actor. type: MainActor
+    static func useGlobal() -> Int {
+      global
+    }
+  }
+
+  _ = Nested.useGlobal()
+}