duplicates:

• SR-4758 SE-0142: derived protocol that restricts associatedtype should be allowed as a variable type

Issue Description:

Frustrating to meet this bug...Since Action is defined (in a mathematical sense), I should be able to use P1 as any other protocol without associated types.

protocol P {
    associatedtype Action

    func handle(_ action: Action)
}

protocol P1: P where Action == Int {}

class Foo: P1 {
    func handle(_ action: Int) {}
}

let foo: P1 = Foo() // Protocol 'P1' can only be used as a generic constraint ...

I understand same-type constraints as semantic refinements rather that defaulting acts. The difference is that the former are irreversible.

protocol P1: P {
    asssociatedtype Action = String
}

is not the same as

protocol P1: P where Action == String

In the first example I specify a default type for Action. This means I can have a different default in an inheriting protocol. Conversely, in the second example I constrain Action to String. In other words, Action becomes a typealias that can't be overridden or changed further in the protocol hierarchy.

However, it is possible to "override" Action even after same-type constraining it, which is very strange to me.

protocol P1: P where Action == Int {}

protocol P2: P1 {     
   associatedtype Action = String 
}

class Foo: P2 {
    func handle(_ action: String) {} // protocol stubs
}

This might be the reason I can't use a protocol with a same-type constraint on it's associated type anywhere other that a generic constraint, or just another bug.