objcProtocol.Type.self fail to cast to Protocol

[miku1958]

Describe the bug
I have a method for manipulating objcProtocol that looks like this

func classConforming<P>(to objcProtocol: P.Type) -> P? {
	let p: Any = objcProtocol
	guard let pp = p as? Protocol else {
		return nil
	}
	return self.objonly_classConformingToProtocol(pp) as? P
}

self.objonly_classConformingToProtocol accepts a Protocol object as parameter and return AnyObject

When I call it like:
classConforming(to: aProtocol.Type.self)
Xcode shows me that aProtocol.Type.self is aProtocol.Type.Protocol and lldb tells me that objcProtocol is "@thick aProtocol.Type.Protocol" and p is "" but print will tell me it is aProtocol.Type and I can't cast it to Protocol, pp will always nil.

I also tried adding where P.Type == Protocol, but the compiler would give me an error:

Generic signature requires types 'P.Type' and 'Protocol' to be the same

I'm not sure how the compiler converts, but I think since Swift lacks a mechanism to determine whether it's a protocol/class/struct/enum/actor, it should at least ensure type consistency when passing parameters

[miku1958]

Another try:
type(of: objcProtocol) will return me “aProtocol.Type.Protocol”, both lldb and print are, but I still can't cast it to Protocol, then I realized that Protocol is a Objc class, so I cast p as? Protocol.Type but pp is still nil

[miku1958]

One possible solution I expect is to make the type returned by aProtocol.self (aProtocol.Protocol, aka Protocol) consistent with the one in the parameter:
I can use P.Protocol instead of Protocol, which is still essentially an Objc object, but then I can return P.Type by genericizing it, like this:

func classConforming<P>(to objcProtocol: P.Protocol) -> P.Type? {
	return self.objonly_classConformingToProtocol(objcProtocol) as? P.Type
}

classConforming(to: aProtocol.self)

[AnthonyLatsis]

What are you trying to do? What is the type signature of objonly_classConformingToProtocol? Casting a metatype value to a non-metatype type will always fail. For example, you cannot cast Array<Int>.self to Collection, because the metatype value itself does not conform to Collection — it is the instances of Array<Int>.self that do.

[miku1958]

Hi @AnthonyLatsis, I know when I use aProtocol.Type.self, the complier will return me a Protocol object, and it will lose all the type information about the aProtocol, and the problem I'm having is that I want to get the Protocol object, and I want the function to be generic, so I can return the object that implements the aProtocol

objonly_classConformingToProtocol takes a Protocol object and returns a Class that can be found in the registry:

- (Class)objonly_classConformingToProtocol:(Protocol *)inProtocol;

[AnthonyLatsis]

The protocol metatype object (the protocol type as an object) is MyProtocol.self (the type of this object is MyProtocol.Protocol). MyProtocol.Type.self is a higher-level, meta-metatype object; its type is MyProtocol.Type.Protocol. Although these meta-metatypes exist as part of the type system, they have little if any practical value.

Unfortunately, there is no way of expressing a generic parameter that accepts only protocol types, so func foo<T>(_: T.Protocol) will not work.

- (Class)objonly_classConformingToProtocol:(Protocol *)inProtocol;

I wasn’t able to find any documentation on this Protocol type. How do you instantiate it? There is no implicit Swift conversion from a protocol metatype like aProtocol.self to Protocol.

[miku1958]

We basically only use the Protocol object as a key to the dictionary to find the class in objonly_classConformingToProtocol, and this practice of using objonly_classConformingToProtocol is very widespread in our projects.

What I mean is, can this generic be added at the compiler level. The compiler can know what I mean when I use P.Protocol like this:

func classConforming<P>(to objcProtocol: P.Protocol) -> P.Type? {
	return self.objonly_classConformingToProtocol(objcProtocol) as? P.Type
}

Or allow me to use Protocol(or protocol) as a generic constraint like:

func classConforming<P>(to objcProtocol: P.Type) -> P.Type? where P: Protocol {
	return self.objonly_classConformingToProtocol(objcProtocol.Protocol) as? P.Type
}

[AnthonyLatsis]

What I mean is, can this generic be added at the compiler level.

No, you cannot constrain a generic parameter to protocols only, Swift currently lacks this kind of expressivity. There is no analogue of : AnyObject for protocols.

func classConforming<P>(to objcProtocol: P.Type) -> P.Type? where P: Protocol {
  return self.objonly_classConformingToProtocol(objcProtocol.Protocol) as? P.Type
}

The question of whether something similar to this will work boils down to the question of how you instantiate an object of this Protocol type in Swift. Have you ever used objonly_classConformingToProtocol in Swift successfully before, without generics? If so, could you provide an example?

[miku1958]

Hi @AnthonyLatsis, I can use objonly_classConformingToProtocol in Swift directly, but the "Class" from Objc will become "AnyClass" in Swift, and for historical reasons, compiler allow me to call any Objc method with AnyClass. It's not safe, and it's not what I want