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[Sema] @objc functions shall not have typed throw #81054

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Merged
DougGregor merged 8 commits into from
Sep 23, 2025
Merged

[Sema] @objc functions shall not have typed throw #81054

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[Sema] Type checking forbids typed throw on @objc functions now
DataCorrupted Apr 23, 2025
cf15d46
Update
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Check for @objc only
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Check typed thrown early
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3 changes: 3 additions & 0 deletions include/swift/AST/DiagnosticsSema.def
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Expand Up @@ -5461,6 +5461,9 @@ 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_thrown_in_objc_forbidden,none,
"@objc functions cannot have typed throw", ())
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@beccadax beccadax Apr 25, 2025

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I would probably word this more similarly to the other diagnostics emitted by isRepresentableInLanguage(), such as not_objc_function_async:

Suggested change
"@objc functions cannot have typed throw", ())
"typed 'throws' %kindonly0 cannot be represented in Objective-C",
(const AbstractFunctionDecl *))

Note that this will insert a string like "instance method" where %kindonly0 is written—you'll need to pass the decl into the diagnose() call, though.

(There are other ways you might word this—if you'd like, take a look around at a few other diagnostics in this method and see if there's something you like better.)


//------------------------------------------------------------------------------
// MARK: Concurrency
//------------------------------------------------------------------------------
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20 changes: 20 additions & 0 deletions lib/Sema/TypeCheckDeclObjC.cpp
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Expand Up @@ -809,6 +809,26 @@ bool swift::isRepresentableInLanguage(
return false;
}
}
auto isTypedThrow = [&]() {
// With no AST token this should be `throws`
if (!AFD->getThrownTypeRepr())
return false;
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@beccadax beccadax Apr 25, 2025

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There's a subtle pitfall here: A TypeRepr is an exact representation of how a type is written in source code, so it's only available when the Swift compiler has directly parsed source code to create the declaration. If a declaration uses typed throws but it was deserialized from a .swiftmodule file or synthesized by the compiler, then getThrownTypeRepr() will return nullptr even though there is actually a typed throws.

The various methods that return the thrown type as a Type, rather than a TypeRepr *, don't have this problem, so you can just remove the getThrownTypeRepr() check—checking getThrownInterfaceType() alone will tell you everything you need to know.

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@DataCorrupted DataCorrupted Apr 26, 2025

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I am fairly new to swift, can you elaborate more on "If a declaration uses typed throws but it was deserialized from a .swiftmodule file or synthesized by the compiler", how to create either case, and how are they used in practice?

// Or it is a `throws(<ErrorType>)`
CanType thrownType = AFD->getThrownInterfaceType()->getCanonicalType();
// 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.
if (thrownType->isErrorExistentialType())
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@beccadax beccadax Apr 25, 2025

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Digging into the implementation underlying getThrownInterfaceType(), it turns out that if a function is non-throwing, it will return the type for Swift.Never. So if you wrote this condition:

Suggested change
if (thrownType->isErrorExistentialType())
if (thrownType->isNever() || thrownType->isErrorExistentialType())

You would not need to explicitly check hasThrows() before calling your isTypedThrow() function.

(If you don't do the isNever() check, you might want to remove the call to getCanonicalType() on line 817—TypeBase::isErrorExistentialType() implicitly canonicalizes the type, so you don't have to.)

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@DataCorrupted DataCorrupted Apr 26, 2025 •
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thrownType could be a null pointer if not getCanonicalType, even if AFD hasThrow. I'm not sure why.

I'd prefer keep the hasThrow check to make the intention more clear, it also made more sense to erase the closure that this point.

return false;
softenIfAccessNote(AFD, Reason.getAttr(),
AFD->diagnose(diag::typed_thrown_in_objc_forbidden)
.limitBehavior(behavior));
Reason.describe(AFD);
return true;
};
if (AFD->hasThrows() && isTypedThrow())
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@beccadax beccadax Apr 25, 2025

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Oh, checking this before the sync and async code paths diverge is clever!

return false;

if (AFD->hasAsync()) {
// Asynchronous functions move all of the result value and thrown error
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12 changes: 12 additions & 0 deletions test/attr/attr_objc.swift
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Expand Up @@ -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
Expand Down Expand Up @@ -2329,6 +2334,13 @@ 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{{@objc functions cannot have typed throw}}
// expected-error@-2{{thrown type 'any MyError' does not conform to the 'Error' protocol}}
@objc func throwsObjCError() throws(ObjCError) {}
// expected-error@-1{{@objc functions cannot have typed throw}}
}


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8 changes: 8 additions & 0 deletions test/attr/attr_objc_async.swift
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Expand Up @@ -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
Expand All @@ -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 {{@objc functions cannot have typed throw}}
}

actor class MyActor2 { }
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