possible implementation of proposed standard library additions, std::value_type and std::value_type_t

value_type.hpp

#include <array>
#include <atomic>
#include <complex>
#include <cstddef>
#include <functional>
#include <list>
#include <memory>
#include <string>
#include <type_traits>
#include <unordered_map>
#include <utility>
#include <vector>

using namespace std;

// https://groups.google.com/a/isocpp.org/forum/#!topic/std-proposals/eINuhhjZgsw

namespace experimental {
	////////////////////
	// value_type<>

	// declaration
	template<class T>
	struct value_type;

	// specialization for C array
	template<class T, int N>
	struct value_type<T[N]> {
		using type = T;
	};

	template<typename T>
	struct value_type {
		using type = typename decay<T>::type::value_type;
	};

	////////////////////
	// value_type_t<>

	template<class T> using value_type_t = typename value_type<T>::type;
}

////////////////////
// test helpers

template<class Actual, class Expected>
struct assert_same {
	static_assert(is_same<Actual, Expected>::value, "test failed");
};

template<class T, class ValueType>
struct test
		: pair<
				// test experimental::value_type		
				assert_same<typename experimental::value_type<T>::type, ValueType>,

				// test experimental::value_type_t		
				assert_same<experimental::value_type_t<T>, ValueType>> {
};

////////////////////
// tests

template
struct test<char[7], char>;

template
struct test<vector<unique_ptr<const string>>&&, unique_ptr<const string>>;

template
struct test<list<reference_wrapper<volatile signed char**>>, reference_wrapper<volatile signed char**>>;

template
struct test<const vector<bool>&, bool>;

template
struct test<const bool[7], const bool>;

template
struct test<complex<float>, float>;

template
struct test<allocator<atomic<long long>>, atomic<long long>>;

template
struct test<array<void*&(double, ...), 0>, void*&(double, ...)>;

template
struct test<
		unordered_map<string, size_t (array<reference_wrapper<short>, 9>::*)>,
		pair<const string, unsigned long array<reference_wrapper<short>, 9>::*>>;