On Tue, 2 Sept 2025 at 09:01, Luc Grosheintz wrote:
>
> As preparation for implementing std::constant_wrapper that's part of the
> C++26 version of the <type_traits> header, the two classes _Index_tuple
> and _Build_index_tuple are moved to <type_traits>. These two helpers are
> needed by std::constant_wrapper to initialize the elements of one C
> array with another.
>
> Since, <bits/utility.h> already includes <type_traits> this solution
> avoids creating a very small header file for just these two internal
> classes.
At some point I'd like to split <type_traits> into smaller pieces. Not
one header per trait, that would be silly (Boost, I'm looking at you),
but we don't need e.g. all the complex invoke_result logic in every
header that wants to use std::decay_t.
But that is a larger project and isn't made any easier or harder by
this small change.
> This approach doesn't move std::index_sequence and related code
> to <type_traits> and therefore doesn't change which headers provide
> user-facing features.
Great, thanks.
OK for trunk.
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/utility.h (_Index_tuple): Move to <type_traits>.
> (_Build_index_tuple): Ditto.
> * include/std/type_traits (_Index_tuple): Ditto.
> (_Build_index_tuple): Ditto.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/bits/utility.h | 20 --------------------
> libstdc++-v3/include/std/type_traits | 22 ++++++++++++++++++++++
> 2 files changed, 22 insertions(+), 20 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/utility.h
> b/libstdc++-v3/include/bits/utility.h
> index 84d25e0c658..4e574658eba 100644
> --- a/libstdc++-v3/include/bits/utility.h
> +++ b/libstdc++-v3/include/bits/utility.h
> @@ -137,26 +137,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> using tuple_element_t = typename tuple_element<__i, _Tp>::type;
> #endif
>
> - // Stores a tuple of indices. Used by tuple and pair, and by bind() to
> - // extract the elements in a tuple.
> - template<size_t... _Indexes> struct _Index_tuple { };
> -
> - // Builds an _Index_tuple<0, 1, 2, ..., _Num-1>.
> - template<size_t _Num>
> - struct _Build_index_tuple
> - {
> -#if __has_builtin(__make_integer_seq)
> - template<typename, size_t... _Indices>
> - using _IdxTuple = _Index_tuple<_Indices...>;
> -
> - // Clang defines __make_integer_seq for this purpose.
> - using __type = __make_integer_seq<_IdxTuple, size_t, _Num>;
> -#else
> - // For GCC and other compilers, use __integer_pack instead.
> - using __type = _Index_tuple<__integer_pack(_Num)...>;
> -#endif
> - };
> -
> #ifdef __glibcxx_integer_sequence // C++ >= 14
>
> /// Class template integer_sequence
> diff --git a/libstdc++-v3/include/std/type_traits
> b/libstdc++-v3/include/std/type_traits
> index ff23544fbf0..4636457eb5a 100644
> --- a/libstdc++-v3/include/std/type_traits
> +++ b/libstdc++-v3/include/std/type_traits
> @@ -4280,6 +4280,28 @@ template<typename _Ret, typename _Fn, typename...
> _Args>
>
> #endif // C++2a
>
> +#if __cplusplus >= 201103L
> + // Stores a tuple of indices. Used by tuple and pair, and by bind() to
> + // extract the elements in a tuple.
> + template<size_t... _Indexes> struct _Index_tuple { };
> +
> + // Builds an _Index_tuple<0, 1, 2, ..., _Num-1>.
> + template<size_t _Num>
> + struct _Build_index_tuple
> + {
> +#if __has_builtin(__make_integer_seq)
> + template<typename, size_t... _Indices>
> + using _IdxTuple = _Index_tuple<_Indices...>;
> +
> + // Clang defines __make_integer_seq for this purpose.
> + using __type = __make_integer_seq<_IdxTuple, size_t, _Num>;
> +#else
> + // For GCC and other compilers, use __integer_pack instead.
> + using __type = _Index_tuple<__integer_pack(_Num)...>;
> +#endif
> + };
> +#endif // C++11
> +
> /// @} group metaprogramming
>
> _GLIBCXX_END_NAMESPACE_VERSION
> --
> 2.51.0
>
