From 64309a3f86fdeab5a60c560a33e108a289200dbc Mon Sep 17 00:00:00 2001
From: Jamie <2119834+jamieQ@users.noreply.github.com>
Date: Tue, 28 Oct 2025 20:30:12 -0500
Subject: [PATCH 1/2] [Sema]: ban the use of 'nonisolated' on actor
 declarations

The combination doesn't make sense, so don't allow it.
---
 lib/Sema/TypeCheckAttr.cpp                        | 9 +++++++++
 test/decl/class/actor/basic.swift                 | 2 ++
 test/decl/protocol/special/DistributedActor.swift | 2 ++
 3 files changed, 13 insertions(+)

diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index b005626ffe527..dbf0ecdfeab69 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -7982,6 +7982,15 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
     }
   }
 
+  // `nonisolated` on an actor declaration is contradictory.
+  if (auto *classDecl = dyn_cast<ClassDecl>(D);
+      classDecl && classDecl->isExplicitActor()) {
+    diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr)
+        .warnUntilFutureSwiftVersion()
+        .fixItRemove(attr->getRange());
+    return;
+  }
+
   if (auto VD = dyn_cast<ValueDecl>(D)) {
     //'nonisolated(unsafe)' is meaningless for computed properties, functions etc.
     auto var = dyn_cast<VarDecl>(VD);
diff --git a/test/decl/class/actor/basic.swift b/test/decl/class/actor/basic.swift
index 23749ee114d15..55c18414014bb 100644
--- a/test/decl/class/actor/basic.swift
+++ b/test/decl/class/actor/basic.swift
@@ -53,3 +53,5 @@ extension A2 {
 
   init(doesNotDelegate: ()) {} // expected-error {{designated initializer cannot be declared in an extension of 'A2'}}
 }
+
+nonisolated actor A3 {} // expected-warning {{'nonisolated' modifier cannot be applied to this declaration; this will be an error in a future Swift language mode}}{{1-13=}}
diff --git a/test/decl/protocol/special/DistributedActor.swift b/test/decl/protocol/special/DistributedActor.swift
index f49ccc399db9c..4390f69b6cadc 100644
--- a/test/decl/protocol/special/DistributedActor.swift
+++ b/test/decl/protocol/special/DistributedActor.swift
@@ -65,6 +65,8 @@ distributed actor D5: P1 {
   // expected-note@-1{{non-distributed instance method 'dist()'}}
 }
 
+nonisolated distributed actor D6 {} // expected-warning {{'nonisolated' modifier cannot be applied to this declaration; this will be an error in a future Swift language mode}}{{1-13=}}
+
 // ==== Tests ------------------------------------------------------------------
 
 // Make sure the conformances have been added implicitly.

From 65a67cdf583a3c0cff617e7e5af92612312299c2 Mon Sep 17 00:00:00 2001
From: Konrad `ktoso` Malawski <konrad.malawski@project13.pl>
Date: Thu, 30 Oct 2025 13:25:55 +0900
Subject: [PATCH 2/2] Make 'nonisolated actor' a hard error

We've searched and can't find any usage of this in the wild, so hope we can get away with banning this outright.
---
 lib/Sema/TypeCheckAttr.cpp                        | 1 -
 test/decl/class/actor/basic.swift                 | 2 +-
 test/decl/protocol/special/DistributedActor.swift | 2 +-
 3 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/Sema/TypeCheckAttr.cpp b/lib/Sema/TypeCheckAttr.cpp
index dbf0ecdfeab69..c44ec26fd85d2 100644
--- a/lib/Sema/TypeCheckAttr.cpp
+++ b/lib/Sema/TypeCheckAttr.cpp
@@ -7986,7 +7986,6 @@ void AttributeChecker::visitNonisolatedAttr(NonisolatedAttr *attr) {
   if (auto *classDecl = dyn_cast<ClassDecl>(D);
       classDecl && classDecl->isExplicitActor()) {
     diagnoseAndRemoveAttr(attr, diag::invalid_decl_modifier, attr)
-        .warnUntilFutureSwiftVersion()
         .fixItRemove(attr->getRange());
     return;
   }
diff --git a/test/decl/class/actor/basic.swift b/test/decl/class/actor/basic.swift
index 55c18414014bb..7a4ffcdbb7cf1 100644
--- a/test/decl/class/actor/basic.swift
+++ b/test/decl/class/actor/basic.swift
@@ -54,4 +54,4 @@ extension A2 {
   init(doesNotDelegate: ()) {} // expected-error {{designated initializer cannot be declared in an extension of 'A2'}}
 }
 
-nonisolated actor A3 {} // expected-warning {{'nonisolated' modifier cannot be applied to this declaration; this will be an error in a future Swift language mode}}{{1-13=}}
+nonisolated actor A3 {} // expected-error {{'nonisolated' modifier cannot be applied to this declaration}}{{1-13=}}
diff --git a/test/decl/protocol/special/DistributedActor.swift b/test/decl/protocol/special/DistributedActor.swift
index 4390f69b6cadc..a3efe499a8390 100644
--- a/test/decl/protocol/special/DistributedActor.swift
+++ b/test/decl/protocol/special/DistributedActor.swift
@@ -65,7 +65,7 @@ distributed actor D5: P1 {
   // expected-note@-1{{non-distributed instance method 'dist()'}}
 }
 
-nonisolated distributed actor D6 {} // expected-warning {{'nonisolated' modifier cannot be applied to this declaration; this will be an error in a future Swift language mode}}{{1-13=}}
+nonisolated distributed actor D6 {} // expected-error {{'nonisolated' modifier cannot be applied to this declaration}}{{1-13=}}
 
 // ==== Tests ------------------------------------------------------------------