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### is_detected

    template<template<class...> class Op, class... Args>
    using is_detected = see-below;

    template<template<class...> class Op, class... Args>
    constexpr bool is_detected_v = is_detected<Op, Args...>::value;

Aliases: If `Op<Args...>` is a valid template-id, aliases true_type, otherwise aliases false_type.

C++ Standard Paper: N4502

Compiler Compatibility: Requires C++11 variadic templates and C++11 template aliases.

Header: `#include <boost/type_traits/is_detected.hpp>`

Examples:

Suppose we wish to "reset" a value of type T, if the type has a `clear()` member function then we should call it, otherwise we should assign a default constructed value:

    template<class T>
    using clear_t = decltype(boost::declval<T&>().clear());

    template<class T>
    void clear(T& value)
    {
        if constexpr (boost::is_detected_v<clear_t, T>) {
            value.clear();
        } else {
            value = T();
        }
    }

See also: is_detected_convertible, is_detected_exact.