Simple metafunction to check if an object is of literal type. Used as an example to show how useful the SFINAE is.

IsLiteral.hxx

#include<type_traits>

// I generally use this metafunction to show how useful the SFINAE is in metaprogramming.

namespace
{
	template<class T>
	struct constexpr_validator
	{
		static constexpr T m_{};
	};

	template<class T>
	constexpr std::integral_constant<bool, (T{}, 1)> test_constexpr(int);

	template<class T>
	constexpr std::false_type test_constexpr(...);
}

constexpr optional(optional&& other) noexcept = delete;
constexpr optional(T&& other) noexcept = delete;