[Bug c++/100157] Support `__type_pack_element` like Clang

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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 <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:58b7dbf865b146a4e65dbda9be6df78f212c03b6

commit r14-92-g58b7dbf865b146a4e65dbda9be6df78f212c03b6
Author: Patrick Palka <ppalka@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 <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.