<tuple>: tuple_cat() should be generalized beyond pair and array

[StephanTLavavej]

When I overhauled tuple_cat() long ago, I special-cased pair and array:

STL/stl/inc/tuple

Lines 870 to 893 in d0d724f

	template <class _Ty, class... _For_array>
	struct _View_as_tuple { // tuple_cat() supports only tuples, pairs, and arrays
	static_assert(_Always_false<_Ty>, "Unsupported tuple_cat arguments.");
	};
	template <class... _Types>
	struct _View_as_tuple<tuple<_Types...>> { // view a tuple as a tuple
	using type = tuple<_Types...>;
	};
	template <class _Ty1, class _Ty2>
	struct _View_as_tuple<pair<_Ty1, _Ty2>> { // view a pair as a tuple
	using type = tuple<_Ty1, _Ty2>;
	};
	template <class _Ty, class... _Types>
	struct _View_as_tuple<array<_Ty, 0>, _Types...> { // view an array as a tuple; ends recursion at 0
	using type = tuple<_Types...>;
	};
	template <class _Ty, size_t _Size, class... _Types>
	struct _View_as_tuple<array<_Ty, _Size>, _Types...>
	: _View_as_tuple<array<_Ty, _Size - 1>, _Ty, _Types...> { // view an array as a tuple; counts down to 0
	};

This is unnecessary. We could simply use tuple_size and tuple_element to accept anything resembling a tuple.

The Standardese is N4835 [tuple.creation]/12 "[Note: An implementation may support additional types in the template parameter pack Tuples that support the tuple-like protocol, such as pair and array. -end note]"