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.

2 files changed, 7 insertions, 0 deletions

diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/utility.h
index abaaae2dd33..4692aa0c9b0 100644
--- a/libstdc++-v3/include/bits/utility.h
+++ b/libstdc++-v3/include/bits/utility.h
@@ -224,6 +224,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #endif // C++17
 #endif // C++14
 
+#if __has_builtin(__type_pack_element)
+  template<size_t _Np, typename... _Types>
+    struct _Nth_type
+    { using type = __type_pack_element<_Np, _Types...>; };
+#else
   template<size_t _Np, typename... _Types>
     struct _Nth_type
     { };
@@ -262,6 +267,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct _Nth_type<1, _Tp0, _Tp1, _Tp2, _Rest...>
     { using type = _Tp1; };
 #endif
+#endif
 
 #if __cplusplus > 202002L
 #define __cpp_lib_ranges_zip 202110L // for <tuple> and <utility>
diff --git a/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc b/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
index 9bce0b1caa2..dd0df9be1ea 100644
--- a/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
+++ b/libstdc++-v3/testsuite/20_util/tuple/element_access/get_neg.cc
@@ -61,3 +61,4 @@ test03()
 
 // { dg-error "tuple index must be in range" "" { target *-*-* } 0 }
 // { dg-prune-output "no type named 'type' in .*_Nth_type" }
+// { dg-prune-output "'__type_pack_element' index is out of range" }