Re: [PATCH 1/4] libstdc++: Heterogeneous std::pair comparisons [PR113386]

[Jonathan Wakely <jwakely@redhat.com>]

Pushed to trunk now.

On Wed, 10 Apr 2024 at 09:51, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> Tested x86_64-linux.
>
> Since this only affects C++20 and later it seems OK for trunk now.
>
> -- >8 --
>
> I'm only treating this as a DR for C++20 for now, because it's less work
> and only requires changes to operator== and operator<=>. To do this for
> older standards would require changes to the six relational operators
> used pre-C++20.
>
> libstdc++-v3/ChangeLog:
>
>         PR libstdc++/113386
>         * include/bits/stl_pair.h (operator==, operator<=>): Support
>         heterogeneous comparisons, as per LWG 3865.
>         * testsuite/20_util/pair/comparison_operators/lwg3865.cc: New
>         test.
> ---
>  libstdc++-v3/include/bits/stl_pair.h          | 32 ++++++++++++++-----
>  .../pair/comparison_operators/lwg3865.cc      | 15 +++++++++
>  2 files changed, 39 insertions(+), 8 deletions(-)
>  create mode 100644 libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc
>
> diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
> index 4f5c8389fa6..45317417c9c 100644
> --- a/libstdc++-v3/include/bits/stl_pair.h
> +++ b/libstdc++-v3/include/bits/stl_pair.h
> @@ -1000,23 +1000,39 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>    template<typename _T1, typename _T2> pair(_T1, _T2) -> pair<_T1, _T2>;
>  #endif
>
> -  /// Two pairs of the same type are equal iff their members are equal.
> -  template<typename _T1, typename _T2>
> +#if __cpp_lib_three_way_comparison && __cpp_lib_concepts
> +  // _GLIBCXX_RESOLVE_LIB_DEFECTS
> +  // 3865. Sorting a range of pairs
> +
> +  /// Two pairs are equal iff their members are equal.
> +  template<typename _T1, typename _T2, typename _U1, typename _U2>
>      inline _GLIBCXX_CONSTEXPR bool
> -    operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
> +    operator==(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y)
>      { return __x.first == __y.first && __x.second == __y.second; }
>
> -#if __cpp_lib_three_way_comparison && __cpp_lib_concepts
> -  template<typename _T1, typename _T2>
> -    constexpr common_comparison_category_t<__detail::__synth3way_t<_T1>,
> -                                          __detail::__synth3way_t<_T2>>
> -    operator<=>(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
> +  /** Defines a lexicographical order for pairs.
> +   *
> +   * For two pairs of comparable types, `P` is ordered before `Q` if
> +   * `P.first` is less than `Q.first`, or if `P.first` and `Q.first`
> +   * are equivalent (neither is less than the other) and `P.second` is
> +   * less than `Q.second`.
> +  */
> +  template<typename _T1, typename _T2, typename _U1, typename _U2>
> +    constexpr common_comparison_category_t<__detail::__synth3way_t<_T1, _U1>,
> +                                          __detail::__synth3way_t<_T2, _U2>>
> +    operator<=>(const pair<_T1, _T2>& __x, const pair<_U1, _U2>& __y)
>      {
>        if (auto __c = __detail::__synth3way(__x.first, __y.first); __c != 0)
>         return __c;
>        return __detail::__synth3way(__x.second, __y.second);
>      }
>  #else
> +  /// Two pairs of the same type are equal iff their members are equal.
> +  template<typename _T1, typename _T2>
> +    inline _GLIBCXX_CONSTEXPR bool
> +    operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
> +    { return __x.first == __y.first && __x.second == __y.second; }
> +
>    /** Defines a lexicographical order for pairs.
>     *
>     * For two pairs of the same type, `P` is ordered before `Q` if
> diff --git a/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc b/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc
> new file mode 100644
> index 00000000000..2bbd54af192
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/pair/comparison_operators/lwg3865.cc
> @@ -0,0 +1,15 @@
> +// { dg-do run { target c++20 } }
> +
> +// LWG 3865. Sorting a range of pairs
> +
> +#include <utility>
> +#include <testsuite_hooks.h>
> +
> +int main()
> +{
> +  std::pair<int, int> p(1, 2);
> +  std::pair<int&, int&> p2(p.first, p.second);
> +  VERIFY( p == p2 );
> +  VERIFY( p <= p2 );
> +  VERIFY( p >= p2 );
> +}
> --
> 2.44.0
>