From b0b23a070bfb18474293d43062df109e57186e0c Mon Sep 17 00:00:00 2001
From: Slava Pestov <spestov@apple.com>
Date: Fri, 20 Jun 2025 14:26:42 -0400
Subject: [PATCH] Sema: Expand isolated conformance inference to consider
 conformances to inherited protocols

Otherwise, if PDerived inherits from P and P's requirements are
witnessed by @MainActor-isolated members, we fail to infer
@MainActor isolation on the conformance to PDerived.

- Fixes https://github.com/swiftlang/swift/issues/82222.
- Fixes rdar://153219831.
---
 lib/Sema/TypeCheckConcurrency.cpp             | 22 ++++++++++++++-----
 .../Inputs/isolated_conformance_other.swift   |  5 +++++
 .../isolated_conformance_inference.swift      |  4 ++++
 ...ormance_inference_with_default_actor.swift |  9 ++++++++
 4 files changed, 35 insertions(+), 5 deletions(-)
 create mode 100644 test/Concurrency/Inputs/isolated_conformance_other.swift
 create mode 100644 test/Concurrency/isolated_conformance_inference_with_default_actor.swift

diff --git a/lib/Sema/TypeCheckConcurrency.cpp b/lib/Sema/TypeCheckConcurrency.cpp
index b65c9a5f36d94..34aed98b49ca1 100644
--- a/lib/Sema/TypeCheckConcurrency.cpp
+++ b/lib/Sema/TypeCheckConcurrency.cpp
@@ -8155,16 +8155,28 @@ ActorIsolation swift::inferConformanceIsolation(
     return nominalIsolation;
   }
 
-  bool anyIsolatedWitness = false;
   auto protocol = conformance->getProtocol();
-  for (auto requirement : protocol->getMembers()) {
-    if (isa<TypeDecl>(requirement))
+
+  // Also check the value witnesses of each implied conformance to every
+  // inherited protocol, recursively.
+  for (auto req : protocol->getRequirementSignature().getRequirements()) {
+    if (req.getKind() != RequirementKind::Conformance ||
+        !req.getFirstType()->isEqual(ctx.TheSelfType))
       continue;
 
-    auto valueReq = dyn_cast<ValueDecl>(requirement);
-    if (!valueReq)
+    auto *assocConf = conformance->getAssociatedConformance(
+        req.getFirstType(), req.getProtocolDecl()).getConcrete();
+    auto isolation = assocConf->getIsolation();
+    if (isolation.isGlobalActor())
+      return isolation;
+  }
+
+  bool anyIsolatedWitness = false;
+  for (auto requirement : protocol->getProtocolRequirements()) {
+    if (isa<TypeDecl>(requirement))
       continue;
 
+    auto valueReq = cast<ValueDecl>(requirement);
     auto witness = conformance->getWitnessDecl(valueReq);
     if (!witness)
       continue;
diff --git a/test/Concurrency/Inputs/isolated_conformance_other.swift b/test/Concurrency/Inputs/isolated_conformance_other.swift
new file mode 100644
index 0000000000000..8b4fa2e88e697
--- /dev/null
+++ b/test/Concurrency/Inputs/isolated_conformance_other.swift
@@ -0,0 +1,5 @@
+public protocol P {
+  func f()
+}
+
+public protocol PDerived: P {}
diff --git a/test/Concurrency/isolated_conformance_inference.swift b/test/Concurrency/isolated_conformance_inference.swift
index d3481c2c651c5..35bf78603a3ba 100644
--- a/test/Concurrency/isolated_conformance_inference.swift
+++ b/test/Concurrency/isolated_conformance_inference.swift
@@ -92,3 +92,7 @@ class InferMeDefaults {
   var someGlobalActorState: any P2.Type = DifferingConformances.self // expected-error{{global actor 'SomeGlobalActor'-isolated default value in a main actor-isolated context}}
   var bothState: any (P & P2).Type = DifferingConformances.self // expected-error{{default argument cannot be both main actor-isolated and global actor 'SomeGlobalActor'-isolated}}
 }
+
+protocol PDerived: P {}
+
+@MainActor struct ImpliedConformanceInference: PDerived { func f() {} }
diff --git a/test/Concurrency/isolated_conformance_inference_with_default_actor.swift b/test/Concurrency/isolated_conformance_inference_with_default_actor.swift
new file mode 100644
index 0000000000000..57a86a58978b7
--- /dev/null
+++ b/test/Concurrency/isolated_conformance_inference_with_default_actor.swift
@@ -0,0 +1,9 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/isolated_conformance_other.swiftmodule %S/Inputs/isolated_conformance_other.swift -swift-version 6
+// RUN: %target-typecheck-verify-swift -I %t -swift-version 6 -enable-upcoming-feature InferIsolatedConformances -default-isolation MainActor -swift-version 6
+// REQUIRES: swift_feature_InferIsolatedConformances
+
+import isolated_conformance_other
+
+struct S1: P { func f() {} }
+struct S2: PDerived { func f() {} }