protocol P {
    typealias T
    
    func f()
    func g()
}

extension P {
    func f() {
        print("P.f")
    }
    
    func g() {
        print("P.g")
        f() // Should call P.Equatable.f if T is Equatable, shouldn't it?
    }
}

extension P where T: Equatable {
    func f() {
        print("P.Equatable.f") // Inaccessible to P.g? Not even to P.Equatable.g but accessible to to Foo.Equatable.g — odd.
    }
    
//    func g() {
//        print("P.Equatable.g")
//        f() // Should call P.Equatable.f if T is Equatable, shouldn't it?
//    }
}

struct Foo<U>: P {
    typealias T = U
    
//    func g() {
//        print("Foo.g")
//        f()
//    }
}

extension Foo where U: Equatable {
//    func f() {
//        print("Foo.Equatable.f") // Inaccessible to P.g or Foo.g? At least accessible to Foo.Equatable.g.
//    }
    
//    func g() {
//        print("Foo.Equatable.g")
//        f()
//    }
}

let foo = Foo<Int>()
foo.f()
print("")
foo.g()

// Uncomment P.Equatable.g, Foo.g, Foo.Equatable.g, Foo.Equatable.f to see other behaviors.

P.Equatable.f

P.g
P.f

P.Equatable.f

P.g
P.Equatable.f