Example of forward-declaring std types

std-fwd.h

// example of forward-declaring std types.
// *** don't use this! it's for exposition only! ***
#pragma once

#ifdef __has_include
	#define STDFWD_HAS_INCLUDE(header)		__has_include(header)
#else
	#define STDFWD_HAS_INCLUDE(header)		0
#endif

#if STDFWD_HAS_INCLUDE(<version>)
	#include <version>
#endif

namespace std
{
	template <typename>				struct	hash;
	template <typename>				struct	pointer_traits;
	template <typename>				struct	tuple_size;
	template <size_t, typename>		struct	tuple_element;
	template <typename, typename>	struct	pair;

	template <typename, size_t>		class	array;
	template <typename>				class	optional;
	template <typename...>			class	tuple;
	#ifdef __cpp_lib_span
	template <typename, size_t>		class	span;
	#endif
}