[PATCH] libstdc++: Implement LWG3528 make_from_tuple can perform (the equivalent of) a C-style cast

[Tomasz Kaminski]

On Mon, Jun 9, 2025 at 4:07 PM Jonathan Wakely <jwakely@redhat.com> wrote:

> From: Yihan Wang <yronglin777@gmail.com>
>
> Implement LWG3528 to make std::make_from_tuple SFINAE friendly.
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/tuple (__can_make_from_tuple): New variable
>         template.
>         (__make_from_tuple_impl): Add static_assert.
>         (make_from_tuple): Constrain using __can_make_from_tuple.
>         * testsuite/20_util/tuple/dr3528.cc: New test.
>
> Signed-off-by: Yihan Wang <yronglin777@gmail.com>
> Co-authored-by: Jonathan Wakely <jwakely@redhat.com>
> ---
>
> Here's a simpler version of Yihan's patch, with constraints on
> std::make_from_tuple but not on __make_from_tuple_impl, and with an
> original test rather than one copied from MSVC and libc++.
>
> Lightly tested on x86_64-linux, full testing underway.
>
LGTM. Thanks.

>
>  libstdc++-v3/include/std/tuple                | 24 +++++++++-
>  .../testsuite/20_util/tuple/dr3528.cc         | 44 +++++++++++++++++++
>  2 files changed, 66 insertions(+), 2 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
>
> diff --git a/libstdc++-v3/include/std/tuple
> b/libstdc++-v3/include/std/tuple
> index 2e69af13a98b..b39ce710984c 100644
> --- a/libstdc++-v3/include/std/tuple
> +++ b/libstdc++-v3/include/std/tuple
> @@ -2939,19 +2939,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>  #endif
>
>  #ifdef __cpp_lib_make_from_tuple // C++ >= 17
> +  template <typename _Tp, typename _Tuple, typename _Seq
> +             =
> make_index_sequence<tuple_size_v<remove_reference_t<_Tuple>>>>
> +    constexpr bool __can_make_from_tuple = false;
> +
> +  // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +  // 3528. make_from_tuple can perform (the equivalent of) a C-style cast
> +  template <typename _Tp, typename _Tuple, size_t... _Idx>
> +    constexpr bool __can_make_from_tuple<_Tp, _Tuple,
> index_sequence<_Idx...>>
> +      = is_constructible_v<_Tp,
> +
> decltype(std::get<_Idx>(std::declval<_Tuple>()))...>;
> +
>    template <typename _Tp, typename _Tuple, size_t... _Idx>
>      constexpr _Tp
>      __make_from_tuple_impl(_Tuple&& __t, index_sequence<_Idx...>)
> -    { return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...); }
> +    {
> +      static_assert(__can_make_from_tuple<_Tp, _Tuple,
> index_sequence<_Idx...>>);
> +      return _Tp(std::get<_Idx>(std::forward<_Tuple>(__t))...);
> +    }
>
>  #if __cpp_lib_tuple_like // >= C++23
>    template <typename _Tp, __tuple_like _Tuple>
>  #else
>    template <typename _Tp, typename _Tuple>
>  #endif
> -    constexpr _Tp
> +    constexpr auto
>      make_from_tuple(_Tuple&& __t)
>      noexcept(__unpack_std_tuple<is_nothrow_constructible, _Tp, _Tuple>)
> +#ifdef __cpp_concepts // >= C++20
> +    -> _Tp
> +    requires __can_make_from_tuple<_Tp, _Tuple>
> +#else
> +    -> __enable_if_t<__can_make_from_tuple<_Tp, _Tuple>, _Tp>
> +#endif
>      {
>        constexpr size_t __n = tuple_size_v<remove_reference_t<_Tuple>>;
>  #if __has_builtin(__reference_constructs_from_temporary)
> diff --git a/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
> b/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
> new file mode 100644
> index 000000000000..a96c6b75e209
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/tuple/dr3528.cc
> @@ -0,0 +1,44 @@
> +// { dg-do compile { target c++17 } }
> +
> +// LWG 3528. make_from_tuple can perform (the equivalent of) a C-style
> cast
> +
> +#include <tuple>
> +#include <array>
> +#include <utility>
> +
> +template<typename T, typename Tuple>
> +using make_t = decltype(std::make_from_tuple<T>(std::declval<Tuple>()));
> +
> +template<typename T, typename Tuple, typename = void>
> +constexpr bool can_make = false;
> +template<typename T, typename Tuple>
> +constexpr bool can_make<T, Tuple, std::void_t<make_t<T, Tuple>>> = true;
> +
> +static_assert( can_make<int, std::tuple<int>> );
> +static_assert( can_make<int, std::tuple<int>&> );
> +static_assert( can_make<int, const std::tuple<int>&> );
> +static_assert( can_make<int, std::array<short, 1>> );
> +static_assert( can_make<int, const std::array<short, 1>&&> );
> +static_assert( can_make<std::tuple<int, int>, std::pair<unsigned, long>>
> );
> +static_assert( can_make<const int*, std::tuple<int*>> );
> +static_assert( can_make<void*, std::tuple<int*>> );
> +static_assert( can_make<int, std::tuple<>> );
> +static_assert( ! can_make<int, std::tuple<int, int>> );
> +static_assert( ! can_make<int, std::pair<short, char>> );
> +static_assert( ! can_make<int, std::pair<short, char>&> );
> +static_assert( ! can_make<int, std::tuple<const char*>> );
> +static_assert( ! can_make<int*, std::tuple<const int*>> );
> +static_assert( ! can_make<int*, std::tuple<void*>> );
> +static_assert( ! can_make<int, std::array<int, 2>> );
> +static_assert( ! can_make<void, std::tuple<>> );
> +static_assert( ! can_make<void, std::array<int, 1>> );
> +
> +struct Two
> +{
> +  Two(const char*, int);
> +};
> +
> +static_assert( can_make<Two, std::tuple<char*, unsigned>> );
> +static_assert( ! can_make<Two, std::tuple<const char*, int, int>> );
> +static_assert( can_make<Two, std::pair<const char*, long>> );
> +static_assert( ! can_make<Two, std::pair<int*, long>> );
> --
> 2.49.0
>
>
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