On Tue, Mar 25, 2025 at 1:43 PM Jonathan Wakely <jwak...@redhat.com> wrote:
> LWG 4229 points out that the std::ranges::to wording refers to class
> types, but I added an assertion using std::is_class_v which only allows
> non-union class types. LWG consensus is that unions should be allowed,
> so this additionally uses std::is_union_v.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/ranges (ranges::to): Allow unions as well as
> non-union class types.
> * testsuite/std/ranges/conv/lwg4229.cc: New test.
> ---
>
> Tested x86_64-linux.
>
LGTM. I still do not have any real case for using ranges::to with unions,
but consensus seems to be to allow them.
>
> libstdc++-v3/include/std/ranges | 4 ++--
> .../testsuite/std/ranges/conv/lwg4229.cc | 18 ++++++++++++++++++
> 2 files changed, 20 insertions(+), 2 deletions(-)
> create mode 100644 libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
>
> diff --git a/libstdc++-v3/include/std/ranges
> b/libstdc++-v3/include/std/ranges
> index 34c6f113e21..7a339c51368 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -9421,7 +9421,7 @@ namespace __detail
> to [[nodiscard]] (_Rg&& __r, _Args&&... __args)
> {
> static_assert(!is_const_v<_Cont> && !is_volatile_v<_Cont>);
> - static_assert(is_class_v<_Cont>);
> + static_assert(is_class_v<_Cont> || is_union_v<_Cont>);
>
> if constexpr (__detail::__toable<_Cont, _Rg>)
> {
> @@ -9580,7 +9580,7 @@ namespace __detail
> to [[nodiscard]] (_Args&&... __args)
> {
> static_assert(!is_const_v<_Cont> && !is_volatile_v<_Cont>);
> - static_assert(is_class_v<_Cont>);
> + static_assert(is_class_v<_Cont> || is_union_v<_Cont>);
>
> using __detail::_To;
> using views::__adaptor::_Partial;
> diff --git a/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
> b/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
> new file mode 100644
> index 00000000000..780ed1fd932
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/std/ranges/conv/lwg4229.cc
> @@ -0,0 +1,18 @@
> +// { dg-do compile { target c++23 } }
> +
> +// LWG 4229 std::ranges::to with union return type
> +
> +#include <ranges>
> +
> +union U
> +{
> + template<std::ranges::input_range R> U(std::from_range_t, R&&) { }
> +
> + int i;
> +};
> +
> +void
> +test_lwg4229(std::ranges::subrange<int*> r)
> +{
> + U u = std::ranges::to<U>(r);
> +}
> --
> 2.49.0
>
>
