c++: Define built-in for std::tuple_element [PR100157]

c++: Define built-in for std::tuple_element [PR100157]

This adds a new built-in to replace the recursive class template
instantiations done by traits such as std::tuple_element and
std::variant_alternative.  The purpose is to select the Nth type from a
list of types, e.g. __type_pack_element<1, char, int, float> is int.
We implement it as a special kind of TRAIT_TYPE.

For a pathological example tuple_element_t<1000, tuple<2000 types...>>
the compilation time is reduced by more than 90% and the memory used by
the compiler is reduced by 97%.  In realistic examples the gains will be
much smaller, but still relevant.

Unlike the other built-in traits, __type_pack_element uses template-id
syntax instead of call syntax and is SFINAE-enabled, matching Clang's
implementation.  And like the other built-in traits, it's not mangleable
so we can't use it directly in function signatures.

N.B. Clang seems to implement __type_pack_element as a first-class
template that can e.g. be used as a template-template argument.  For
simplicity we implement it in a more ad-hoc way.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
	PR c++/100157

gcc/cp/ChangeLog:

	* cp-trait.def (TYPE_PACK_ELEMENT): Define.
	* cp-tree.h (finish_trait_type): Add complain parameter.
	* cxx-pretty-print.cc (pp_cxx_trait): Handle
	CPTK_TYPE_PACK_ELEMENT.
	* parser.cc (cp_parser_constant_expression): Document default
	arguments.
	(cp_parser_trait): Handle CPTK_TYPE_PACK_ELEMENT.  Pass
	tf_warning_or_error to finish_trait_type.
	* pt.cc (tsubst) <case TRAIT_TYPE>: Handle non-type first
	argument.  Pass complain to finish_trait_type.
	* semantics.cc (finish_type_pack_element): Define.
	(finish_trait_type): Add complain parameter.  Handle
	CPTK_TYPE_PACK_ELEMENT.
	* tree.cc (strip_typedefs): Handle non-type first argument.
	Pass tf_warning_or_error to finish_trait_type.
	* typeck.cc (structural_comptypes) <case TRAIT_TYPE>: Use
	cp_tree_equal instead of same_type_p for the first argument.

libstdc++-v3/ChangeLog:

	* include/bits/utility.h (_Nth_type): Conditionally define in
	terms of __type_pack_element if available.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Prune
	additional errors from the new built-in.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/type_pack_element1.C: New test.
	* g++.dg/ext/type_pack_element2.C: New test.
	* g++.dg/ext/type_pack_element3.C: New test.