Check for existence of nested type alias and conditionally set type alias

Question

I wonder how to conditionally set a type alias, based on the existance of a type alias in input argument like this.

struct a { using type = int; }

template <typename T> struct wrapper {
    using inner_t = ???how???; // if T::type exists, use T::type, else T
};

static_assert(std::is_same<wrapper<int>::inner_t, int>, "expected int");
static_assert(std::is_same<wrapper<a>::inner_t, int>, "expected inner type: int");

One naive attempt is like std::conditional<std::is_class<T>, T::type, T> if the existence of a type alias is a prerequisite, but is far from being safe, lacking a check for something like is_alias_present<T, type>. Besides that T::type is potentially not existing for all types resulting in a compiler error.

Apearently std::experimental::is_detected looks promising, but unfortunately not an option for now.

Not sure if metaprogramming libraries like boost::mp11 or boost::hana are a good approach to get that done.

Answer 1

There are 2 things, the traits to detect presence, and the lazy evaluation.

In C++20, the traits can be done easily with requires

template <typename T>
concept has_type = requires { typename T::type; }

Then the lazy evaluation:

template <typename T> struct wrapper {
    using inner_t = std::conditional_t<has_type<T>, T, std::type_identity<T> >::type;
};