https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100157
--- Comment #12 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Patrick Palka <ppa...@gcc.gnu.org>: https://gcc.gnu.org/g:58b7dbf865b146a4e65dbda9be6df78f212c03b6 commit r14-92-g58b7dbf865b146a4e65dbda9be6df78f212c03b6 Author: Patrick Palka <ppa...@redhat.com> Date: Wed Apr 19 15:36:34 2023 -0400 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 <jwak...@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.
