std::experimental::is_detected, std::experimental::detected_t, std::experimental::detected_or

From cppreference.com

Defined in header `<experimental/type_traits>`
template< template<class...> class Op, class... Args > using is_detected = /* see below */;		(library fundamentals TS v2)
template< template<class...> class Op, class... Args > using detected_t = /* see below */;		(library fundamentals TS v2)
template< class Default, template<class...> class Op, class... Args > using detected_or = /* see below */;		(library fundamentals TS v2)

The alias template detected_or is an alias for an unspecified class type with two public member typedefs value_t and type, which are defined as follows:

If the template-id Op<Args...> is valid, then value_t is an alias for std::true_type, and type is an alias for Op<Args...>;
Otherwise, value_t is an alias for std::false_type and type is an alias for Default.

The alias template is_detected is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::value_t. It is an alias for std::true_type if the template-id Op<Args...> is valid; otherwise it is an alias for std::false_type.

The alias template detected_t is equivalent to typename detected_or<std::experimental::nonesuch, Op, Args...>::type. It is an alias for Op<Args...> if that template-id is valid; otherwise it is an alias for the class std::experimental::nonesuch.

Additional utilities

template< template<class...> class Op, class... Args > constexpr bool is_detected_v = is_detected<Op, Args...>::value;		(library fundamentals TS v2)
template< class Default, template<class...> class Op, class... Args > using detected_or_t = typename detected_or<Default, Op, Args...>::type;		(library fundamentals TS v2)
template <class Expected, template<class...> class Op, class... Args> using is_detected_exact = is_same<Expected, detected_t<Op, Args...>>;		(library fundamentals TS v2)
template <class Expected, template<class...> class Op, class... Args> constexpr bool is_detected_exact_v = is_detected_exact<Expected, Op, Args...>::value;		(library fundamentals TS v2)
template <class To, template<class...> class Op, class... Args> using is_detected_convertible = is_convertible<detected_t<Op, Args...>, To>;		(library fundamentals TS v2)
template <class To, template<class...> class Op, class... Args> constexpr bool is_detected_convertible_v = is_detected_convertible<To, Op, Args...>::value;		(library fundamentals TS v2)

The alias template is_detected_exact checks whether detected_t<Op, Args...> is Expected.

The alias template is_detected_convertible checks whether detected_t<Op, Args...> is convertible to To.

Possible implementation

namespace detail {
template <class Default, class AlwaysVoid,
          template<class...> class Op, class... Args>
struct detector {
  using value_t = std::false_type;
  using type = Default;
};
 
template <class Default, template<class...> class Op, class... Args>
struct detector<Default, std::void_t<Op<Args...>>, Op, Args...> {
  // Note that std::void_t is a C++17 feature
  using value_t = std::true_type;
  using type = Op<Args...>;
};
 
} // namespace detail
 
template <template<class...> class Op, class... Args>
using is_detected = typename detail::detector<nonesuch, void, Op, Args...>::value_t;
 
template <template<class...> class Op, class... Args>
using detected_t = typename detail::detector<nonesuch, void, Op, Args...>::type;
 
template <class Default, template<class...> class Op, class... Args>
using detected_or = detail::detector<Default, void, Op, Args...>;

Example

Run this code

#include <experimental/type_traits>
 
template<class T>
using copy_assign_t = decltype(std::declval<T&>() = std::declval<const T&>());
 
struct Meow { };
struct Purr { void operator=(const Purr&) = delete; };
 
int main()
{
    static_assert(std::experimental::is_detected<copy_assign_t, Meow>::value, "Meow should be copy assignable!");
    static_assert(!std::experimental::is_detected_v<copy_assign_t, Purr>, "Purr should not be copy assignable!");
    static_assert(std::experimental::is_detected_exact_v<Meow&, copy_assign_t, Meow>, "Copy assignment of Meow should return Meow&!");
}