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;