std::experimental::is_detected,std::experimental::detected_t,(3)

NAME
std::experimental::is_detected,std::experimental::detected_t, - std::experimental::is_detected,std::experimental::detected_t,

Synopsis
   Defined in header <experimental/type_traits>
   template< template<class...> class Op, class... Args > (library fundamentals TS v2)
   using is_detected = /* see below */;
   template< template<class...> class Op, class... Args > (library fundamentals TS v2)
   using detected_t = /* see below */;
   template< class Default, template<class...> class Op,
   class... Args > (library fundamentals TS v2)
   using detected_or = /* see below */;
   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...> denotes a valid type, 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...> denotes a valid type; 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 denotes a valid type; otherwise it is an alias for
   the class std::experimental::nonesuch.
   Additional utilities
   template< template<class...> class Op, class... Args > (library fundamentals TS
   constexpr bool is_detected_v = is_detected<Op, v2)
   Args...>::value;
   template< template<class...> class Op, class... Args > (library fundamentals TS
   constexpr inline bool is_detected_v = is_detected<Op, v3)
   Args...>::value;
   template< class Default, template<class...> class Op,
   class... Args > (library fundamentals TS
   using detected_or_t = typename detected_or<Default, Op, v2)
   Args...>::type;
   template< class Expected, template<class...> class Op,
   class... Args > (library fundamentals TS
   using is_detected_exact = std::is_same<Expected, v2)
   detected_t<Op, Args...>>;
   template< class Expected, template<class...> class Op,
   class... Args >
                        (library fundamentals TS
   constexpr bool is_detected_exact_v = v2)
       is_detected_exact<Expected, Op, Args...>::value;
   template< class Expected, template<class...> class Op,
   class... Args >
                        (library fundamentals TS
   constexpr inline bool is_detected_exact_v = v3)
       is_detected_exact<Expected, Op, Args...>::value;
   template< class To, template<class...> class Op, class...
   Args >
                        (library fundamentals TS
   using is_detected_convertible = v2)
       std::is_convertible<detected_t<Op, Args...>, To>;
   template< class To, template<class...> class Op, class...
   Args >
                        (library fundamentals TS
   constexpr bool is_detected_convertible_v = v2)
       is_detected_convertible<To, Op, Args...>::value;
   template< class To, template<class...> class Op, class...
   Args >
                        (library fundamentals TS
   constexpr inline bool is_detected_convertible_v = v3)
       is_detected_convertible<To, Op, Args...>::value;
   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...>
     {
         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 <cstddef>
 #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; };
 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&!");
 template<class T>
 using diff_t = typename T::difference_type;
 template<class Ptr>
 using difference_type = std::experimental::detected_or_t<std::ptrdiff_t, diff_t, Ptr>;
 struct Woof { using difference_type = int; };
 struct Bark {};
 static_assert(std::is_same<difference_type<Woof>, int>::value,
               "Woof's difference_type should be int!");
 static_assert(std::is_same<difference_type<Bark>, std::ptrdiff_t>::value,
               "Bark's difference_type should be ptrdiff_t!");
 int main() {}