From a4a39f2a98c3258d0b962a8f06dc526718201f41 Mon Sep 17 00:00:00 2001
From: Doug Gregor <dgregor@apple.com>
Date: Tue, 15 Jul 2025 13:00:45 -0700
Subject: [PATCH] [SE-0466] Nested types of nonisolated types don't infer
 main-actor isolation

Under discussion as part of an amendment to SE-0466, limit default main
actor inference so it doesn't apply to a nested type within a
nonisolated type.
---
 lib/Sema/TypeCheckConcurrency.cpp             | 21 ++++++++++++++++---
 ...tor_typechecker_errors_sendablecheck.swift |  7 ++++++-
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index 86b6670b2a4cb..a13432f2e20b9 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -6104,13 +6104,28 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
         -> std::optional<std::tuple<InferredActorIsolation, ValueDecl *,
                                     std::optional<ActorIsolation>>> {
       // Default global actor isolation does not apply to any declarations
-      // within actors and distributed actors.
-      bool inActorContext = false;
+      // within actors and distributed actors, nor does it apply in a
+      // nonisolated type.
       auto *dc = value->getInnermostDeclContext();
-      while (dc && !inActorContext) {
+      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:
+              break;
+            }
+          }
         }
         dc = dc->getParent();
       }
diff --git a/test/Concurrency/assume_mainactor_typechecker_errors_sendablecheck.swift b/test/Concurrency/assume_mainactor_typechecker_errors_sendablecheck.swift
index e85ca00f762f2..2b258a768a785 100644
--- a/test/Concurrency/assume_mainactor_typechecker_errors_sendablecheck.swift
+++ b/test/Concurrency/assume_mainactor_typechecker_errors_sendablecheck.swift
@@ -9,8 +9,13 @@ enum CK: CodingKey {
   case one
 
   func f() { }
+
+  struct Nested {
+    func g() { }
+  }
 }
 
-nonisolated func testCK(x: CK) {
+nonisolated func testCK(x: CK, y: CK.Nested) {
   x.f() // okay, because CK and CK.f are not @MainActor.
+  y.g()
 }