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From: "vittorio.romeo at outlook dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/100157] New: Support `__type_pack_element` like Clang
Date: Tue, 20 Apr 2021 17:44:52 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100157

            Bug ID: 100157
           Summary: Support `__type_pack_element` like Clang
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vittorio.romeo at outlook dot com
  Target Milestone: ---

Clang provides a `__type_pack_element` builtin which allows efficient index=
ing
of parameter packs in variadic templates, and it seems that GCC has no
equivalent.

This forces users interested in minimizing compilation times to resort to
arcane implemenations such as these ones:
https://github.com/kvasir-io/mpl/blob/development/src/kvasir/mpl/sequence/l=
ookup.hpp

A builtin like `__type_pack_element` would not only allow user code to comp=
ile
faster, but also anything inside libstdc++ that needs to index a type list
(e.g. `std::tuple_element_t`) would benefit from it.

This is the Clang test driver, to show an usage example:
https://github.com/llvm-mirror/clang/blob/master/test/SemaCXX/type_pack_ele=
ment.cpp

This is the Clang pull request:
https://reviews.llvm.org/D15421=