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

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    Library fundamentals v2
     
    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...> 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 >
constexpr bool is_detected_v = is_detected<Op, Args...>::value;
    		(library fundamentals TS v2) 
    template< template<class...> class Op, class... Args >
constexpr inline bool is_detected_v = is_detected<Op, Args...>::value;
    		(library fundamentals TS v3) 
    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 = std::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 Expected, template<class...> class Op, class... Args >
constexpr inline bool is_detected_exact_v =
    is_detected_exact<Expected, Op, Args...>::value;
    		(library fundamentals TS v3) 
    template< class To, template<class...> class Op, class... Args >
using is_detected_convertible =
    std::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) 
    template< class To, template<class...> class Op, class... Args >
constexpr inline bool is_detected_convertible_v =
    is_detected_convertible<To, Op, Args...>::value;
    		(library fundamentals TS v3)

    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() {}