From 60c711d4a65d3e15356832fba9d27d2579f2d768 Mon Sep 17 00:00:00 2001
From: Allan Shortlidge <shortlidge@apple.com>
Date: Sun, 26 Oct 2025 21:22:41 -0700
Subject: [PATCH] Sema: Fix a regression in retroactive conformance diagnostic
 suppression.

Module qualifying a protocol name in an inheritance clause should suppress
retroactive conformance diagnostics for the protocol and any protocols it
inherits from, just like adding the `@retroactive` attribute. Fixes a
regression in Swift 6.2 introduced by
https://github.com/swiftlang/swift/pull/81576.

Resolves rdar://162295268 and https://github.com/swiftlang/swift/issues/85153.
---
 lib/Sema/TypeCheckDeclPrimary.cpp                  |  9 ++++-----
 test/Sema/extension_retroactive_conformances.swift | 12 ++++++++++--
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index 5bb7d285379c1..dc778b70ba234 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -1864,6 +1864,7 @@ static void diagnoseRetroactiveConformances(
       continue;
     }
 
+    bool isMarkedRetroactive = entry.isRetroactive();
     SmallVector<ProtocolDecl *, 2> protos;
     if (auto *protoTy = inheritedTy->getAs<ProtocolType>()) {
       auto *proto = protoTy->getDecl();
@@ -1871,10 +1872,8 @@ static void diagnoseRetroactiveConformances(
       // As a fallback, to support previous language versions, also allow
       // this through if the protocol has been explicitly module-qualified.
       TypeRepr *repr = unwrapAttributedRepr(entry.getTypeRepr());
-      if (isModuleQualified(repr, proto->getParentModule())) {
-        protocolsWithRetroactiveAttr.insert(proto);
-        continue;
-      }
+      if (isModuleQualified(repr, proto->getParentModule()))
+        isMarkedRetroactive = true;
 
       protos.push_back(proto);
     } else if (auto *compositionTy = inheritedTy->getAs<ProtocolCompositionType>()) {
@@ -1920,7 +1919,7 @@ static void diagnoseRetroactiveConformances(
         }
 
         // If it's marked @retroactive, no need to warn.
-        if (entry.isRetroactive()) {
+        if (isMarkedRetroactive) {
           // Note that we encountered this protocol through a conformance marked
           // @retroactive in case multiple clauses cause the protocol to be
           // inherited.
diff --git a/test/Sema/extension_retroactive_conformances.swift b/test/Sema/extension_retroactive_conformances.swift
index 02d7baf8e7223..fabc0e6bb404e 100644
--- a/test/Sema/extension_retroactive_conformances.swift
+++ b/test/Sema/extension_retroactive_conformances.swift
@@ -20,8 +20,12 @@ public struct Sample3 {}
 public struct Sample4 {}
 public struct Sample5 {}
 public struct Sample6 {}
+public struct Sample6a {}
+public struct Sample6b {}
+public struct Sample6c {}
 public struct Sample7 {}
 public struct Sample8 {}
+public struct Sample8a {}
 
 public struct SampleAlreadyConforms: SampleProtocol1 {}
 
@@ -76,9 +80,13 @@ typealias MySample6 = Sample6
 extension MySample6: SampleProtocol1 {} // expected-warning {{extension declares a conformance of imported type 'Sample6' to imported protocol 'SampleProtocol1'}}
 // expected-note @-1 {{add '@retroactive' to silence this warning}} {{22-37=@retroactive SampleProtocol1}}
 
-// Ensure module-qualifying both types still silences the warning
+// Ensure module-qualifying the protocol silences the warning
 
-extension Library.Sample6: Library.SampleProtocol2 {} // ok, module-qualified.
+extension Library.Sample6: Library.SampleProtocol2 {} // ok, both types are module-qualified.
+extension Sample6a: Library.SampleProtocol2 {} // ok, protocol is module qualified.
+extension Library.Sample6b: SampleProtocol2 {} // expected-warning {{extension declares a conformance of imported type 'Sample6b' to imported protocol 'SampleProtocol2'; this will not behave correctly if the owners of 'Library' introduce this conformance in the future}}
+// expected-note @-1 {{add '@retroactive' to silence this warning}}
+extension Sample6c: Library.SampleProtocol1a {} // ok, protocol is module qualified.
 
 protocol ClientProtocol {}