Checking whether member function is constexpr

is_constexpr_memfn.cc

//Checking whether member function is constexpr

#include <array>
#include <vector>

template<typename T, typename = void>
struct has_constexpr_size : std::false_type {};

template<typename T, typename Fn, typename ...Args>
constexpr auto invoke_memfn(Fn fn, Args...args) { return (T().*fn)(args...); }

template<typename T>
#ifndef _MSC_VER
struct has_constexpr_size<T, std::enable_if_t<(T().size(),true)>> : std::true_type {};
#else
struct has_constexpr_size<T, std::enable_if_t<(invoke_memfn<T>(&T::size),true)>> : std::true_type {};
#endif

template<typename T>
constexpr bool has_constexpr_size_v = has_constexpr_size<T>::value;

static_assert(has_constexpr_size_v<std::array<int, 5>>,"");
static_assert(!has_constexpr_size_v<std::vector<int>>,"");