Accept type A or B<A>

gistfile1.swift

import Foundation

protocol Type {
    typealias Value: Type
}

extension String: Type {
    typealias Value = String
}

struct Attribute<T: Type>: Type {
    typealias Value = T
    let t: T
}

func aFution<T1: Type, T2: Type, V: Type where T1.Value == V, T2.Value == V>(t1: T1, t2: T2) {
    
}

aFution("a", t2: "b")
aFution(Attribute(t: "a"), t2: "b")
aFution("a", t2: Attribute(t: "b"))
aFution(Attribute(t: "a"), t2: Attribute(t: "b"))