[SE-0306] Make actors semantically "final".

Treat actors as being semantically `final` throughout the type checker.
This allows, for example, a non-`required` initializer to satisfy a
protocol requirement.

We're leaving the ABI open for actor inheritance should we need it.

Addresses rdar://78269551.

Author: DougGregor

include/swift/AST/Decl.h

@@ -2374,6 +2374,10 @@ class ValueDecl : public Decl {
   /// Note whether this declaration is known to be exposed to Objective-C.
   void setIsObjC(bool Value);
 
+  /// Is this declaration semantically 'final', meaning that the type checker
+  /// should treat it as final even if the ABI does not?
+  bool isSemanticallyFinal() const;
+
   /// Is this declaration 'final'?
   bool isFinal() const;
 

lib/AST/ASTContext.cpp

@@ -2828,7 +2828,7 @@ AnyFunctionType::Param swift::computeSelfParam(AbstractFunctionDecl *AFD,
     if (Ctx.isSwiftVersionAtLeast(5)) {
       if (wantDynamicSelf && CD->isConvenienceInit())
         if (auto *classDecl = selfTy->getClassOrBoundGenericClass())
-          if (!classDecl->isFinal())
+          if (!classDecl->isSemanticallyFinal())
             isDynamicSelf = true;
     }
   } else if (isa<DestructorDecl>(AFD)) {

lib/AST/Decl.cpp

@@ -2868,6 +2868,23 @@ void ValueDecl::setIsObjC(bool value) {
   LazySemanticInfo.isObjC = value;
 }
 
+bool ValueDecl::isSemanticallyFinal() const {
+  // Actor types are semantically final.
+  if (auto classDecl = dyn_cast<ClassDecl>(this)) {
+    if (classDecl->isActor())
+      return true;
+  }
+
+  // As are members of actor types.
+  if (auto classDecl = getDeclContext()->getSelfClassDecl()) {
+    if (classDecl->isActor())
+      return true;
+  }
+
+  // For everything else, the same as 'final'.
+  return isFinal();
+}
+
 bool ValueDecl::isFinal() const {
   return evaluateOrDefault(getASTContext().evaluator,
                            IsFinalRequest { const_cast<ValueDecl *>(this) },
@@ -3212,13 +3229,13 @@ bool ValueDecl::shouldHideFromEditor() const {
 static AccessLevel getMaximallyOpenAccessFor(const ValueDecl *decl) {
   // Non-final classes are considered open to @testable importers.
   if (auto cls = dyn_cast<ClassDecl>(decl)) {
-    if (!cls->isFinal())
+    if (!cls->isSemanticallyFinal())
       return AccessLevel::Open;
 
   // Non-final overridable class members are considered open to
   // @testable importers.
   } else if (decl->isPotentiallyOverridable()) {
-    if (!cast<ValueDecl>(decl)->isFinal())
+    if (!cast<ValueDecl>(decl)->isSemanticallyFinal())
       return AccessLevel::Open;
   }
 

lib/IDE/CodeCompletion.cpp

@@ -5422,7 +5422,7 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
       switch (Reason) {
       case DeclVisibilityKind::MemberOfProtocolConformedToByCurrentNominal:
       case DeclVisibilityKind::MemberOfProtocolDerivedByCurrentNominal:
-        if (!C->isFinal())
+        if (!C->isSemanticallyFinal())
           needRequired = true;
         break;
       case DeclVisibilityKind::MemberOfSuper:
@@ -5458,7 +5458,7 @@ class CompletionOverrideLookup : public swift::VisibleDeclConsumer {
     if (D->shouldHideFromEditor())
       return;
 
-    if (D->isFinal())
+    if (D->isSemanticallyFinal())
       return;
 
     bool hasIntroducer = hasFuncIntroducer ||

lib/Sema/CSSimplify.cpp

@@ -7720,7 +7720,7 @@ static bool isNonFinalClass(Type type) {
     type = dynamicSelf->getSelfType();
 
   if (auto classDecl = type->getClassOrBoundGenericClass())
-    return !classDecl->isFinal();
+    return !classDecl->isSemanticallyFinal();
 
   if (auto archetype = type->getAs<ArchetypeType>())
     if (auto super = archetype->getSuperclass())

lib/Sema/DerivedConformanceCodable.cpp

@@ -1789,7 +1789,7 @@ static ValueDecl *deriveDecodable_init(DerivedConformance &derived) {
   }
 
   // This constructor should be marked as `required` for non-final classes.
-  if (classDecl && !classDecl->isFinal()) {
+  if (classDecl && !classDecl->isSemanticallyFinal()) {
     auto *reqAttr = new (C) RequiredAttr(/*IsImplicit=*/true);
     initDecl->getAttrs().add(reqAttr);
   }

lib/Sema/DerivedConformances.cpp

@@ -545,7 +545,7 @@ bool DerivedConformance::checkAndDiagnoseDisallowedContext(
   // A non-final class can't have an protocol-witnesss initializer in an
   // extension.
   if (auto CD = dyn_cast<ClassDecl>(Nominal)) {
-    if (!CD->isFinal() && isa<ConstructorDecl>(synthesizing) &&
+    if (!CD->isSemanticallyFinal() && isa<ConstructorDecl>(synthesizing) &&
         isa<ExtensionDecl>(ConformanceDecl)) {
       ConformanceDecl->diagnose(
           diag::cannot_synthesize_init_in_extension_of_nonfinal,

lib/Sema/TypeCheckAttr.cpp

@@ -4489,7 +4489,7 @@ IndexSubset *DifferentiableAttributeTypeCheckRequest::evaluate(
     if (diagnoseDynamicSelfResult) {
       // Diagnose class initializers in non-final classes.
       if (isa<ConstructorDecl>(original)) {
-        if (!classDecl->isFinal()) {
+        if (!classDecl->isSemanticallyFinal()) {
           diags.diagnose(
               attr->getLocation(),
               diag::differentiable_attr_nonfinal_class_init_unsupported,
@@ -4790,7 +4790,7 @@ static bool typeCheckDerivativeAttr(ASTContext &Ctx, Decl *D,
     if (diagnoseDynamicSelfResult) {
       // Diagnose class initializers in non-final classes.
       if (isa<ConstructorDecl>(originalAFD)) {
-        if (!classDecl->isFinal()) {
+        if (!classDecl->isSemanticallyFinal()) {
           diags.diagnose(attr->getLocation(),
                          diag::derivative_attr_nonfinal_class_init_unsupported,
                          classDecl->getDeclaredInterfaceType());

lib/Sema/TypeCheckConcurrency.cpp

@@ -3545,7 +3545,7 @@ bool swift::checkSendableConformance(
 
   if (classDecl) {
     // An non-final class cannot conform to `Sendable`.
-    if (!classDecl->isFinal()) {
+    if (!classDecl->isSemanticallyFinal()) {
       classDecl->diagnose(diag::concurrent_value_nonfinal_class,
                           classDecl->getName())
           .limitBehavior(behavior);

lib/Sema/TypeCheckDecl.cpp

@@ -871,7 +871,7 @@ IsDynamicRequest::evaluate(Evaluator &evaluator, ValueDecl *decl) const {
     return true;
 
   // The presence of 'final' blocks the inference of 'dynamic'.
-  if (decl->isFinal())
+  if (decl->isSemanticallyFinal())
     return false;
 
   // Types are never 'dynamic'.
@@ -1868,7 +1868,7 @@ FunctionOperatorRequest::evaluate(Evaluator &evaluator, FuncDecl *FD) const {
   if (dc->isTypeContext()) {
     if (auto classDecl = dc->getSelfClassDecl()) {
       // For a class, we also need the function or class to be 'final'.
-      if (!classDecl->isFinal() && !FD->isFinal() &&
+      if (!classDecl->isSemanticallyFinal() && !FD->isFinal() &&
           FD->getStaticLoc().isValid() &&
           FD->getStaticSpelling() != StaticSpellingKind::KeywordStatic) {
         FD->diagnose(diag::nonfinal_operator_in_class,

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1028,7 +1028,7 @@ static void checkOverrideAccessControl(ValueDecl *baseDecl, ValueDecl *decl,
   } else if (baseHasOpenAccess &&
              classDecl->hasOpenAccess(dc) &&
              decl->getFormalAccess() < AccessLevel::Public &&
-             !decl->isFinal()) {
+             !decl->isSemanticallyFinal()) {
     {
       auto diag = diags.diagnose(decl, diag::override_not_accessible,
                                  /*setter*/false,
@@ -1150,7 +1150,7 @@ bool OverrideMatcher::checkOverride(ValueDecl *baseDecl,
   if (decl->getASTContext().isSwiftVersionAtLeast(5) &&
       baseDecl->getInterfaceType()->hasDynamicSelfType() &&
       !decl->getInterfaceType()->hasDynamicSelfType() &&
-      !classDecl->isFinal()) {
+      !classDecl->isSemanticallyFinal()) {
     diags.diagnose(decl, diag::override_dynamic_self_mismatch);
     diags.diagnose(baseDecl, diag::overridden_here);
   }
@@ -1942,7 +1942,7 @@ static bool checkSingleOverride(ValueDecl *override, ValueDecl *base) {
   }
 
   // The overridden declaration cannot be 'final'.
-  if (base->isFinal() && !isAccessor) {
+  if (base->isSemanticallyFinal() && !isAccessor) {
     // Use a special diagnostic for overriding an actor's unownedExecutor
     // method.  TODO: only if it's implicit?  But then we need to
     // propagate implicitness in module interfaces.

lib/Sema/TypeCheckProtocol.cpp

@@ -947,7 +947,7 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
   ClassDecl *covariantSelf = nullptr;
   if (witness->getDeclContext()->getExtendedProtocolDecl()) {
     if (auto *classDecl = dc->getSelfClassDecl()) {
-      if (!classDecl->isFinal()) {
+      if (!classDecl->isSemanticallyFinal()) {
         // If the requirement's type does not involve any associated types,
         // we use a class-constrained generic parameter as the 'Self' type
         // in the witness thunk.
@@ -3238,7 +3238,7 @@ printRequirementStub(ValueDecl *Requirement, DeclContext *Adopter,
                      Type AdopterTy, SourceLoc TypeLoc, raw_ostream &OS) {
   if (isa<ConstructorDecl>(Requirement)) {
     if (auto CD = Adopter->getSelfClassDecl()) {
-      if (!CD->isFinal() && isa<ExtensionDecl>(Adopter)) {
+      if (!CD->isSemanticallyFinal() && isa<ExtensionDecl>(Adopter)) {
         // In this case, user should mark class as 'final' or define
         // 'required' initializer directly in the class definition.
         return false;
@@ -4180,7 +4180,7 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
     }
 
     if (auto *classDecl = Adoptee->getClassOrBoundGenericClass()) {
-      if (!classDecl->isFinal()) {
+      if (!classDecl->isSemanticallyFinal()) {
         checkNonFinalClassWitness(requirement, witness);
       }
     }
@@ -5201,7 +5201,7 @@ TypeChecker::couldDynamicallyConformToProtocol(Type type, ProtocolDecl *Proto,
 
   // A non-final class might have a subclass that conforms to the protocol.
   if (auto *classDecl = type->getClassOrBoundGenericClass()) {
-    if (!classDecl->isFinal())
+    if (!classDecl->isSemanticallyFinal())
       return true;
   }
 

test/Concurrency/global_actor_inference.swift

@@ -175,10 +175,10 @@ actor GenericSuper<T> {
 
 actor GenericSub<T> : GenericSuper<[T]> { // expected-error{{actor types do not support inheritance}}
   override func method() { }  // expected-note {{calls to instance method 'method()' from outside of its actor context are implicitly asynchronous}}
-  // expected-error@-1{{actor-isolated instance method 'method()' has different actor isolation from global actor 'GenericGlobalActor<[T]>'-isolated overridden declaration}}
+  // expected-error@-1{{instance method overrides a 'final' instance method}}
 
-  @GenericGlobalActor<T> override func method2() { } // expected-error{{global actor 'GenericGlobalActor<T>'-isolated instance method 'method2()' has different actor isolation from global actor 'GenericGlobalActor<[T]>'-isolated overridden declaration}}
-  nonisolated override func method3() { } // expected-error{{nonisolated instance method 'method3()' has different actor isolation from global actor 'GenericGlobalActor<[T]>'-isolated overridden declaration}}
+  @GenericGlobalActor<T> override func method2() { } // expected-error{{instance method overrides a 'final' instance method}}
+  nonisolated override func method3() { } // expected-error{{instance method overrides a 'final' instance method}}
 
   @OtherGlobalActor func testMethod() {
     method() // expected-error{{actor-isolated instance method 'method()' can not be referenced from global actor 'OtherGlobalActor'}}

test/ModuleInterface/actor_objc.swift

@@ -13,6 +13,6 @@
 import Foundation
 
 // CHECK-LABEL: @objc @_inheritsConvenienceInitializers public actor SomeActor : ObjectiveC.NSObject {
-// CHECK: @objc override dynamic public init()
+// CHECK: @objc override public init()
 public actor SomeActor: NSObject {
 }

test/decl/class/actor/conformance.swift

@@ -53,3 +53,29 @@ actor OtherActor: SyncProtocol {
   static func staticMethod() { }
   static var staticProperty: Int = 17
 }
+
+protocol Initializers {
+  init()
+  init(string: String)
+  init(int: Int) async
+}
+
+protocol SelfReqs {
+  func withBells() async -> Self
+}
+
+actor A1: Initializers, SelfReqs {
+  init() { }
+  init(string: String) { }
+  init(int: Int) async { }
+
+  func withBells() async -> A1 { self }
+}
+
+actor A2: Initializers {
+  init() { }
+  init(string: String) { }
+  init(int: Int) { }
+
+  func withBells() async -> A2 { self }
+}