1

Given the following code snippet from an unmodifiable source

protocol P {}

protocol D {
    func foo<K: P>(_: K.Type)
}

extension Int: D {
    func foo<K: P>(_: K.Type) { print("Int P") } // <0>
}

I wonder if there is a way to provide a more specific overload of foo(_:) by means of extending the protocol D. In detail, I'd like to add a new protocol Q and an overload for when K: P & Q (or, more generally, a new constraint on K):

protocol Q {}
struct S: P & Q {}

extension D {
    func foo<K: P & Q>(_: K.Type) { print("P & Q") } // <1>
}

0.foo(S.self) // prints "Int P", not "P & Q"

I can get the desired behavior only if I use S directly in the function signature:

extension D {
    func foo(_: S.Type) { print("S") } // <2>
}

0.foo(S.self) // prints "S"

Is this intended behavior? Intuitively, I would assume that either both <1> and <2> take precedence over <0> or none does.

2

This issue sounds similar if not an exact match to this issue. This response is the relevant one seems to explain the reason:

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