[Sema] @objc functions shall not have typed throw

include/swift/AST/DiagnosticsSema.def

@@ -5461,6 +5461,10 @@ WARNING(no_throw_in_do_with_catch,none,
 ERROR(thrown_type_not_error,none,
       "thrown type %0 does not conform to the 'Error' protocol", (Type))
 
+ERROR(typed_throw_in_objc_forbidden,none,
+      "typed 'throws' %kindonly0 cannot be represented in Objective-C",
+      (const AbstractFunctionDecl *))
+
 //------------------------------------------------------------------------------
 // MARK: Concurrency
 //------------------------------------------------------------------------------

lib/Sema/TypeCheckDeclObjC.cpp

@@ -810,6 +810,23 @@ bool swift::isRepresentableInLanguage(
     }
   }
 
+  // Check that @objc functions can't have typed throw.
+  if (AFD->hasThrows()) {
+    Type thrownType = AFD->getThrownInterfaceType();
+    // TODO: only `throws(Error)` is allowed.
+    // Throwing `any MyError` that confronts `Error` is not implemented yet.
+    // Shall we allow `any MyError` in the future, we should check against
+    // `isExistentialType` instead, and other type checks will make sure it
+    // confrons to `Error`.
+    if (thrownType && !thrownType->isErrorExistentialType()) {
+      softenIfAccessNote(AFD, Reason.getAttr(),
+                         AFD->diagnose(diag::typed_throw_in_objc_forbidden, AFD)
+                             .limitBehavior(behavior));
+      Reason.describe(AFD);
+      return false;
+    }
+  }
+
   if (AFD->hasAsync()) {
     // Asynchronous functions move all of the result value and thrown error
     // information into a completion handler.

test/attr/attr_objc.swift

@@ -2268,6 +2268,11 @@ extension Protocol_ObjC1 {
 //===---
 //===--- Error handling
 //===---
+enum ObjCError: Int, Error {
+  case Others
+}
+protocol MyError: Error { }
+
 // CHECK-LABEL: class ClassThrows1
 class ClassThrows1 {
   // CHECK: @objc func methodReturnsVoid() throws
@@ -2329,6 +2334,15 @@ class ClassThrows1 {
 
   // CHECK: {{^}} func fooWithErrorProtocolComposition2(x: any Error & Protocol_ObjC1)
   func fooWithErrorProtocolComposition2(x: Error & Protocol_ObjC1) { }
+
+  @objc func throwsError() throws(Error) {}
+  @objc func throwsMyError() throws(MyError) {}
+  // expected-error@-1{{typed 'throws' instance method cannot be represented in Objective-C}}
+  // expected-error@-2{{thrown type 'any MyError' does not conform to the 'Error' protocol}}
+  @objc func throwsObjCError() throws(ObjCError) {}
+  // expected-error@-1{{typed 'throws' instance method cannot be represented in Objective-C}}
+  @objc static func throwsObjCErrorClass() throws(ObjCError) {}
+  // expected-error@-1{{typed 'throws' static method cannot be represented in Objective-C}}
 }
 
 

test/attr/attr_objc_async.swift

@@ -27,6 +27,10 @@ class MyClass {
 
 // actor exporting Objective-C entry points.
 
+enum ObjCError: Int, Error {
+  case Others
+}
+
 // CHECK: actor MyActor
 actor MyActor {
   // CHECK: @objc func doBigJobActor() async -> Int
@@ -49,6 +53,10 @@ actor MyActor {
 
   // CHECK: @objc nonisolated func synchronousGood()
   @objc nonisolated func synchronousGood() { }
+
+  @objc func objcAsyncThrowsError() async throws(Error) -> Void {}
+  @objc func objcAsyncThrowsObjCError() async throws(ObjCError) -> Void {}
+  // expected-error@-1 {{typed 'throws' instance method cannot be represented in Objective-C}}
 }
 
 actor class MyActor2 { }