protocol A {
    static func foo()
}
func callFoo<T: A>(_: T.Type) {
    print("** callfoo")
    print(T.self)
    T.foo()
}

extension Dictionary: A where Key == Value {
    static func foo() {
        print("** Dictionary.foo")
        print(type(of: self))
    }
}

extension Array where Element: A {
    static func foo() {
        print("** Array.foo")
        print(type(of: self))
        print(Element.self)
        Element.foo()
        callFoo(Element.self)
    }
}

[String: String].foo()
[[String: String]].foo()

** Dictionary.foo
Dictionary<String, String>.Type
** Array.foo
Array<Dictionary<String, String>>.Type
Dictionary<String, String>
** Dictionary.foo
Dictionary<Dictionary<String, String>, Dictionary<String, String>>.Type
** callfoo
Dictionary<String, String>
** Dictionary.foo
Dictionary<Dictionary<String, String>, Dictionary<String, String>>.Type