You call it with:	Your function prototype is:	template<typename T> void func(T val)	template<typename T> void func(T& val)	template<typename T> void func(const T& val)	template<typename T> void func(T&& val)
(Tested with GCC 7.2 using -std=c++17)
int x; func(x);	T is	int	int	int	int&
	decltype(val) is	int	int&	const int&	int&
	decltype((val)) is	int&	int&	const int&	int&
const int y; func(y);	T is	int	const int	int	const int&
	decltype(val) is	int	const int&	const int&	const int&
	decltype((val)) is	int&	const int&	const int&	const int&
func(5);	T is	int	⛔	int	int
	decltype(val) is	int		const int&	int&&
	decltype((val)) is	int&		const int&	int&
func(std::move(x));	T is	int	⛔	int	int
	decltype(val) is	int		const int&	int&&
	decltype((val)) is	int&		const int&	int&
int&& z = std::move(x); func(z);	T is	int	int	int	int&
	decltype(val) is	int	int&	const int&	int&
	decltype((val)) is	int&	int&	const int&	int&
int x; func<int>(x);	T is	int	int	int	⛔
	decltype(val) is	int	int&	const int&
	decltype((val)) is	int&	int&	const int&
const int y; func<int>(y);	T is	int	⛔	int	⛔
	decltype(val) is	int		const int&
	decltype((val)) is	int&		const int&
func<int>(5);	T is	int	⛔	int	int
	decltype(val) is	int		const int&	int&&
	decltype((val)) is	int&		const int&	int&
int x; func<int&>(x);	T is	int&	int&	int&	int&
	decltype(val) is	int&	int&	int&	int&
	decltype((val)) is	int&	int&	int&	int&
const int y; func<int&>(y);	T is	⛔	⛔	⛔	⛔
	decltype(val) is
	decltype((val)) is
func<int&>(5);	T is	⛔	⛔	⛔	⛔
	decltype(val) is
	decltype((val)) is
int x; func<int&&>(x);	T is	⛔	int&&	int&&	⛔
	decltype(val) is		int&	int&
	decltype((val)) is		int&	int&
const int y; func<int&&>(y);	T is	⛔	⛔	⛔	⛔
	decltype(val) is
	decltype((val)) is
func<int&&>(5);	T is	int&&	⛔	⛔	int&&
	decltype(val) is	int&&			int&&
	decltype((val)) is	int&			int&
int x; func<const int>(x);	T is	const int	const int	const int	⛔
	decltype(val) is	const int	const int&	const int&
	decltype((val)) is	const int&	const int&	const int&
const int y; func<const int>(y);	T is	const int	const int	const int	⛔
	decltype(val) is	const int	const int&	const int&
	decltype((val)) is	const int&	const int&	const int&
func<const int>(5);	T is	const int	const int	const int	const int
	decltype(val) is	const int	const int&	const int&	const int&&
	decltype((val)) is	const int&	const int&	const int&	const int&
int x; func<const int&>(x);	T is	const int&	const int&	const int&	const int&
	decltype(val) is	const int&	const int&	const int&	const int&
	decltype((val)) is	const int&	const int&	const int&	const int&
const int y; func<const int&>(y);	T is	const int&	const int&	const int&	const int&
	decltype(val) is	const int&	const int&	const int&	const int&
	decltype((val)) is	const int&	const int&	const int&	const int&
func<const int&>(5);	T is	const int&	const int&	const int&	const int&
	decltype(val) is	const int&	const int&	const int&	const int&
	decltype((val)) is	const int&	const int&	const int&	const int&
int x; func<const int&&>(x);	T is	⛔	const int&&	const int&&	⛔
	decltype(val) is		const int&	const int&
	decltype((val)) is		const int&	const int&
const int y; func<const int&&>(y);	T is	⛔	const int&&	const int&&	⛔
	decltype(val) is		const int&	const int&
	decltype((val)) is		const int&	const int&
func<const int&&>(5);	T is	const int&&	const int&&	⛔	const int&&
	decltype(val) is	const int&&	const int&		const int&&
	decltype((val)) is	const int&	const int&		const int&