On Fri, 24 Oct 2025 at 12:38, Matthias Kretz <[email protected]> wrote:
>
> Tested on x86_64-linux. (Not tested with Clang yet.) OK for trunk?
>
> --------------------- 8< --------------------
>
> This patch introduces the internal helper type _IndexPack to simplify
> defining a pack of indices via a structured binding declaration:
> constexpr auto [...__is] = _IndexPack<N>;
>
> _IndexPack is a distinct object to enable the lowest overhead in terms
> of template instantiations. Non-GCC compilers that do not implement
> __integer_pack have a slightly higher overhead.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/utility.h (_IndexPack): Define.
> * testsuite/ext/indexpack.cc: New test.
>
> Signed-off-by: Matthias Kretz <[email protected]>
> ---
> libstdc++-v3/include/bits/utility.h | 14 ++++++++++++++
> libstdc++-v3/testsuite/ext/indexpack.cc | 20 ++++++++++++++++++++
> 2 files changed, 34 insertions(+)
> create mode 100644 libstdc++-v3/testsuite/ext/indexpack.cc
>
> diff --git a/libstdc++-v3/include/bits/utility.h b/libstdc++-v3/include/bits/
> utility.h
> index 4e574658eba..6ebcdbe08ef 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -170,6 +170,20 @@ struct integer_sequence
> /// Alias template index_sequence_for
> template<typename... _Types>
> using index_sequence_for = make_index_sequence<sizeof...(_Types)>;
> +
> +#if __has_builtin(__integer_pack)
> + template <auto _Num, typename _Tp = decltype(_Num)>
> + inline constexpr _Tp
> + _IndexPack[_Num] = {__integer_pack(_Tp(_Num))...};
> +#else
> + template <auto _Num, typename _Tp = decltype(_Num), typename =
> make_integer_sequence<_Tp, _Num>>
> + inline constexpr _Tp
> + _IndexPack[_Num];
> +
> + template <auto _Num, typename _Tp, _Tp... _Is>
> + inline constexpr _Tp
> + _IndexPack<_Num, _Tp, integer_sequence<_Tp, _Is...>>[_Num] = {_Is...};
> +#endif // __integer_pack
> #endif // __glibcxx_integer_sequence
This is defined for C++14 and later, but I think it can't actually be
used for anything useful without C++26 packs introduced by structured
bindings, right?
I wonder if we want to restrict the definition of _IndexPack to C++26?
>
> #if __cplusplus >= 201703L
> diff --git a/libstdc++-v3/testsuite/ext/indexpack.cc b/libstdc++-v3/testsuite/
> ext/indexpack.cc
> new file mode 100644
> index 00000000000..bd835fdbc4b
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/ext/indexpack.cc
> @@ -0,0 +1,20 @@
> +// { dg-do compile { target c++26 } }
> +
> +#include <utility>
> +#include <type_traits>
> +
> +template<auto N>
> +void test()
> +{
> + constexpr auto [id0, ...ids] = std::_IndexPack<N>;
> + static_assert( std::is_same_v<decltype(id0), const decltype(N)> );
> + static_assert( sizeof...(ids) == N - 1 );
> + static_assert( (id0 + ... + ids) == N*(N-1)/2 );
> +}
> +
> +int main()
> +{
> + test<1>();
> + test<4u>();
> + test<8ull>();
> +}
> --
> ──────────────────────────────────────────────────────────────────────────
> Dr. Matthias Kretz https://mattkretz.github.io
> GSI Helmholtz Center for Heavy Ion Research https://gsi.de
> std::simd
> ──────────────────────────────────────────────────────────────────────────
>
