From 546f66b1059a63a1caf17033b98e5138402334ab Mon Sep 17 00:00:00 2001
From: Konrad Malawski <ktoso@apple.com>
Date: Mon, 8 Dec 2025 11:30:12 +0900
Subject: [PATCH] [Concurrency] warn about 'final' on actors being ineffective

---
 include/swift/AST/DiagnosticsSema.def                  | 3 +++
 lib/Sema/TypeCheckDeclPrimary.cpp                      | 8 +++++++-
 test/Distributed/distributed_actor_basic.swift         | 2 ++
 test/Interop/SwiftToCxx/class/swift-actor-in-cxx.swift | 2 +-
 4 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/swift/AST/DiagnosticsSema.def b/include/swift/AST/DiagnosticsSema.def
index 116e11f825d1e..8b633708d456b 100644
--- a/include/swift/AST/DiagnosticsSema.def
+++ b/include/swift/AST/DiagnosticsSema.def
@@ -5699,6 +5699,9 @@ ERROR(async_objc_dynamic_self,none,
 ERROR(actor_inheritance,none,
       "%select{actor|distributed actor}0 types do not support inheritance",
       (bool))
+WARNING(actor_final_no_effect,none,
+        "'final' on %kind0 has no effect",
+        (const ValueDecl*))
 
 ERROR(actor_protocol_illegal_inheritance,none,
       "non-actor type %0 cannot conform to the %1 protocol",
diff --git a/lib/Sema/TypeCheckDeclPrimary.cpp b/lib/Sema/TypeCheckDeclPrimary.cpp
index 6c3c4466a2d99..9e9ce1f5f998c 100644
--- a/lib/Sema/TypeCheckDeclPrimary.cpp
+++ b/lib/Sema/TypeCheckDeclPrimary.cpp
@@ -3284,9 +3284,15 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
     TypeChecker::checkDeclAttributes(CD);
 
-    if (CD->isActor())
+    if (CD->isActor()) {
       TypeChecker::checkConcurrencyAvailability(CD->getLoc(), CD);
 
+      if (CD->isFinal()) {
+        CD->diagnose(diag::actor_final_no_effect, CD)
+            .fixItRemove(CD->getAttrs().getAttribute<FinalAttr>()->getLocation());
+      }
+    }
+
     for (Decl *Member : CD->getABIMembers())
       visit(Member);
 
diff --git a/test/Distributed/distributed_actor_basic.swift b/test/Distributed/distributed_actor_basic.swift
index 868e169298484..85d8c32b5a67e 100644
--- a/test/Distributed/distributed_actor_basic.swift
+++ b/test/Distributed/distributed_actor_basic.swift
@@ -23,3 +23,5 @@ distributed actor Second {
     try! await first.one(second: self)
   }
 }
+
+final distributed actor WarnAboutMe { } // expected-warning{{'final' on distributed actor 'WarnAboutMe' has no effect}}
diff --git a/test/Interop/SwiftToCxx/class/swift-actor-in-cxx.swift b/test/Interop/SwiftToCxx/class/swift-actor-in-cxx.swift
index 52e5c26cd8564..f2f6d1f4910d7 100644
--- a/test/Interop/SwiftToCxx/class/swift-actor-in-cxx.swift
+++ b/test/Interop/SwiftToCxx/class/swift-actor-in-cxx.swift
@@ -16,7 +16,7 @@
 // REQUIRES: concurrency
 
 @_expose(Cxx)
-public final actor ActorWithField {
+public final actor ActorWithField { // expected-warning{{'final' on actor 'ActorWithField' has no effect}}
   var field: Int64
 
   public init() {