Re: [PATCH] libstdc++: Fix fallout from P2321R2 pair/tuple enhancements

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 24 Aug 2022 at 18:00, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> On Wed, 24 Aug 2022, Patrick Palka wrote:
>
> > r13-2159-g72886fcc626953 introduced some testsuite regressions in C++23
> > mode:
> >
> >   FAIL: 20_util/pair/requirements/explicit_instantiation/1.cc (test for excess errors)
> >   FAIL: 20_util/tuple/53648.cc (test for excess errors)
> >   FAIL: 20_util/tuple/cons/noexcept_specs.cc (test for excess errors)
> >   FAIL: 20_util/tuple/requirements/explicit_instantiation.cc (test for excess errors)
> >
> > The test noexcept_specs.cc just needs to be updated to consider the
> > additional converting constructors of tuple in C++23 mode, which happen
> > to improve constructing from a const tuple rvalue containing rvalue
> > reference elements (for the better, as far as I can tell).
> >
> > The other three tests fail because they explicitly instantiate a specialization
> > of tuple whose elements are not all const swappable, which in C++23 mode now
> > results in a hard error due to tuple's new const swap member function.  Rather
> > than XFAILing the tests in C++23 mode, this patch adds non-standard constraints
> > to this member function so that we continue to accept such explicit instantiations.
> >
> > Tested on x86_64-pc-linux-gnu with and without --target_board=unix/-std=gnu++23,
> > does this look OK for trunk?
> >
> > libstdc++-v3/ChangeLog:
> >
> >       * include/std/tuple (tuple::swap const): Add is_swappable_v constraints.
>
> D'oh, pair's const swap needs the same workaround:

OK for trunk, thanks for the quick fix.

>
> -- >8 --
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/stl_pair.h (pair::swap const): Add non-standard
>         is_swappable_v constraints.
>         * include/std/tuple (tuple::swap const): Likewise.
>         * testsuite/20_util/tuple/cons/noexcept_specs.cc: Update some
>         asserts in C++23 mode.
> ---
>  libstdc++-v3/include/bits/stl_pair.h          |  8 ++++
>  libstdc++-v3/include/std/tuple                |  9 ++++
>  .../20_util/tuple/cons/noexcept_specs.cc      | 41 +++++++++++++++++++
>  3 files changed, 58 insertions(+)
>
> diff --git a/libstdc++-v3/include/bits/stl_pair.h b/libstdc++-v3/include/bits/stl_pair.h
> index bffca0daf65..a65a4763c54 100644
> --- a/libstdc++-v3/include/bits/stl_pair.h
> +++ b/libstdc++-v3/include/bits/stl_pair.h
> @@ -213,10 +213,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        }
>
>  #if __cplusplus > 202002L
> +      // As an extension, we constrain the const swap member function
> +      // in order to continue accepting explicit instantiation of tuple
> +      // specializations whose elements are not all const swappable.
> +      // Without this constraint, such an explicit instantiation would
> +      // also instantiate the ill-formed body of this function and yield
> +      // a hard error.  This constraint shouldn't affect the behavior of
> +      // valid programs.
>        constexpr void
>        swap(const pair& __p) const
>        noexcept(__and_v<__is_nothrow_swappable<const _T1>,
>                        __is_nothrow_swappable<const _T2>>)
> +      requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
>        {
>         using std::swap;
>         swap(first, __p.first);
> diff --git a/libstdc++-v3/include/std/tuple b/libstdc++-v3/include/std/tuple
> index 05433d5ae36..9d1abc2f80e 100644
> --- a/libstdc++-v3/include/std/tuple
> +++ b/libstdc++-v3/include/std/tuple
> @@ -1176,9 +1176,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        { _Inherited::_M_swap(__in); }
>
>  #if __cplusplus > 202002L
> +      // As an extension, we constrain this const swap member function
> +      // in order to continue accepting explicit instantiation of tuple
> +      // specializations whose elements are not all const swappable.
> +      // Without this constraint, such an explicit instantiation would
> +      // also instantiate the ill-formed body of this function and
> +      // yield a hard error.  This constraint shouldn't affect the behavior of
> +      // valid programs.
>        constexpr void
>        swap(const tuple& __in) const
>        noexcept(__and_v<__is_nothrow_swappable<const _Elements>...>)
> +      requires (is_swappable_v<const _Elements> && ...)
>        { _Inherited::_M_swap(__in); }
>  #endif // C++23
>      };
> @@ -1730,6 +1738,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>        swap(const tuple& __in) const
>        noexcept(__and_v<__is_nothrow_swappable<const _T1>,
>                        __is_nothrow_swappable<const _T2>>)
> +      requires is_swappable_v<const _T1> && is_swappable_v<const _T2>
>        { _Inherited::_M_swap(__in); }
>  #endif // C++23
>      };
> diff --git a/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc b/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
> index 6044a377348..a326d1bc228 100644
> --- a/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
> +++ b/libstdc++-v3/testsuite/20_util/tuple/cons/noexcept_specs.cc
> @@ -503,7 +503,17 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(!std::is_nothrow_constructible<BT, IT&>::value, "");
>    static_assert(!std::is_nothrow_constructible<BT, const IT &>::value, "");
>    static_assert(!std::is_nothrow_constructible<BT, std::tuple<int&>>::value, "");
> +#if __cplusplus > 202002L
> +  // C++23 extended tuple's constructor overload set as part of P2321R2, after
> +  // which its converting constructors more accurately forward the elements
> +  // from a non-const tuple lvalue and from a const tuple rvalue.  In particular
> +  // for the below test we now forward int&& as an rvalue reference instead of
> +  // as an lvalue reference, which means we now select the noexcept B(int&&)
> +  // ctor instead of the non-noexcept B(const int&) ctor.
> +  static_assert(std::is_nothrow_constructible<BT, const std::tuple<int&&>>::value, "");
> +#else
>    static_assert(!std::is_nothrow_constructible<BT, const std::tuple<int&&>>::value, "");
> +#endif
>
>    static_assert(test_trait::is_nothrow_convertible<int,BT>::value,"");
>    static_assert(!test_trait::is_nothrow_convertible<const int,BT>::value,"");
> @@ -515,7 +525,13 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(!test_trait::is_nothrow_convertible<IT&,BT>::value,"");
>    static_assert(!test_trait::is_nothrow_convertible<const IT &,BT>::value,"");
>    static_assert(!test_trait::is_nothrow_convertible<std::tuple<int&>,BT>::value,"");
> +#if __cplusplus > 202002L
> +  // See the note about P2321R2 above.
> +  static_assert(test_trait::is_nothrow_convertible<const std::tuple<int&&>,BT>::value,"");
> +#else
>    static_assert(!test_trait::is_nothrow_convertible<const std::tuple<int&&>,BT>::value,"");
> +#endif
> +
>
>
>    static_assert(!std::is_nothrow_constructible<BT, B>::value, "");
> @@ -528,7 +544,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(std::is_nothrow_constructible<BT, BT&>::value, "");
>    static_assert(std::is_nothrow_constructible<BT, const BT &>::value, "");
>    static_assert(std::is_nothrow_constructible<BT, std::tuple<B&>>::value, "");
> +#if __cplusplus > 202002L
> +  // See the note about P2321R2 above.
> +  static_assert(!std::is_nothrow_constructible<BT, const std::tuple<B&&>>::value, "");
> +#else
>    static_assert(std::is_nothrow_constructible<BT, const std::tuple<B&&>>::value, "");
> +#endif
>
>    static_assert(!test_trait::is_nothrow_convertible<B,BT>::value,"");
>    static_assert(test_trait::is_nothrow_convertible<const B,BT>::value,"");
> @@ -540,7 +561,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(test_trait::is_nothrow_convertible<BT&,BT>::value,"");
>    static_assert(test_trait::is_nothrow_convertible<const BT &,BT>::value,"");
>    static_assert(test_trait::is_nothrow_convertible<std::tuple<B&>,BT>::value,"");
> +#if __cplusplus > 202002L
> +  // See the note about P2321R2 above.
> +  static_assert(!test_trait::is_nothrow_convertible<const std::tuple<B&&>,BT>::value,"");
> +#else
>    static_assert(test_trait::is_nothrow_convertible<const std::tuple<B&&>,BT>::value,"");
> +#endif
>
>  /* explicit */
>    static_assert(std::is_nothrow_constructible<DT, int>::value, "");
> @@ -553,7 +579,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(!std::is_nothrow_constructible<DT, IT&>::value, "");
>    static_assert(!std::is_nothrow_constructible<DT, const IT &>::value, "");
>    static_assert(!std::is_nothrow_constructible<DT, std::tuple<int&>>::value, "");
> +#if __cplusplus > 202002L
> +  // See the note about P2321R2 above.
> +  static_assert(std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
> +#else
>    static_assert(!std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
> +#endif
>
>    static_assert(!std::is_nothrow_constructible<DT, D>::value, "");
>    static_assert(std::is_nothrow_constructible<DT,const D>::value, "");
> @@ -565,7 +596,12 @@ namespace NothrowCopyThrowMoveThrowCopyConversionNothrowMoveConversion
>    static_assert(std::is_nothrow_constructible<DT, DT&>::value, "");
>    static_assert(std::is_nothrow_constructible<DT, const DT &>::value, "");
>    static_assert(std::is_nothrow_constructible<DT, std::tuple<D&>>::value, "");
> +#if __cplusplus > 202002L
> +  // See note about P2321R2 above.
> +  static_assert(!std::is_nothrow_constructible<DT, const std::tuple<D&&>>::value, "");
> +#else
>    static_assert(std::is_nothrow_constructible<DT, const std::tuple<D&&>>::value, "");
> +#endif
>
>    static_assert(!test_trait::is_nothrow_convertible<DT,DT>::value,"");
>    static_assert(test_trait::is_nothrow_convertible<const DT,DT>::value,"");
> @@ -884,7 +920,12 @@ namespace ThrowMoveNothrowConversion
>    static_assert(std::is_nothrow_constructible<DT, IT&>::value, "");
>    static_assert(std::is_nothrow_constructible<DT, const IT &>::value, "");
>    static_assert(std::is_nothrow_constructible<DT, std::tuple<int&>>::value, "");
> +#if __cplusplus > 202002L
> +  // See the note about P2321R2 above.
> +  static_assert(!std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
> +#else
>    static_assert(std::is_nothrow_constructible<DT, const std::tuple<int&&>>::value, "");
> +#endif
>
>    static_assert(test_trait::is_nothrow_convertible<DT,DT>::value,"");
>    static_assert(test_trait::is_nothrow_convertible<D,DT>::value,"");
> --
> 2.37.2.382.g795ea8776b
>