[SE-0316] Non-final classes cannot be global actors

(cherry picked from commit 83050d6)

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4545,6 +4545,8 @@ ERROR(global_actor_on_local_variable,none,
       "local variable %0 cannot have a global actor", (DeclName))
 ERROR(global_actor_non_unsafe_init,none,
       "global actor attribute %0 argument can only be '(unsafe)'", (Type))
+ERROR(global_actor_non_final_class,none,
+      "non-final class %0 cannot be a global actor", (DeclName))
 
 ERROR(actor_isolation_multiple_attr,none,
       "%0 %1 has multiple actor-isolation attributes ('%2' and '%3')",

lib/Sema/TypeCheckConcurrency.cpp

@@ -313,8 +313,17 @@ VarDecl *GlobalActorInstanceRequest::evaluate(
     return nullptr;
   }
 
+  // Non-final classes cannot be global actors.
+  if (auto classDecl = dyn_cast<ClassDecl>(nominal)) {
+    if (!classDecl->isSemanticallyFinal()) {
+      nominal->diagnose(diag::global_actor_non_final_class, nominal->getName())
+        .highlight(globalActorAttr->getRangeWithAt());
+    }
+  }
+
   // Global actors have a static property "shared" that provides an actor
-  // instance. The value must
+  // instance. The value must be of Actor type, which is validated by
+  // conformance to the 'GlobalActor' protocol.
   SmallVector<ValueDecl *, 4> decls;
   nominal->lookupQualified(
       nominal, DeclNameRef(ctx.Id_shared), NL_QualifiedDefault, decls);

test/attr/global_actor.swift

@@ -20,7 +20,7 @@ struct GenericGlobalActor<T> {
 
 // Ill-formed global actors.
 @globalActor
-open class GA2 { // expected-error{{type 'GA2' does not conform to protocol 'GlobalActor'}}
+final class GA2 { // expected-error{{type 'GA2' does not conform to protocol 'GlobalActor'}}
 }
 
 @globalActor
@@ -35,7 +35,7 @@ struct GA4 {
 }
 
 @globalActor
-open class GA5 {
+open class GA5 { // expected-error{{non-final class 'GA5' cannot be a global actor}}
   static let shared = SomeActor() // expected-error{{property 'shared' must be declared public because it matches a requirement in public protocol 'GlobalActor'}}
   // expected-note@-1{{mark the static property as 'public' to satisfy the requirement}}
 }
@@ -49,7 +49,7 @@ extension GA6 where T: Equatable {
 }
 
 @globalActor
-class GA7 { // expected-error{{type 'GA7' does not conform to protocol 'GlobalActor'}}
+final class GA7 { // expected-error{{type 'GA7' does not conform to protocol 'GlobalActor'}}
   static let shared = 5 // expected-note{{candidate would match and infer 'ActorType' = 'Int' if 'Int' conformed to 'Actor'}}
 }