Re: [PATCH] libstdc++: Make views::single/iota/istream SFINAE-friendly [PR108362]

[Jonathan Wakely <jwakely@redhat.com>]

On Wed, 8 Mar 2023 at 14:36, Patrick Palka via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> Tested on x86_64-pc-linux-gnu, does this look OK for trunk and perhaps 12?

Yes, OK for trunk and 12.

>
>         PR libstdc++/108362
>
> libstdc++-v3/ChangeLog:
>
>         * include/std/ranges (__detail::__can_single_view): New concept.
>         (_Single::operator()): Constrain it.  Move [[nodiscard]] to the
>         end of the function declarator.
>         (__detail::__can_iota_view): New concept.
>         (_Iota::operator()): Constrain it.  Move [[nodiscard]] to the
>         end of the function declarator.
>         (__detail::__can_istream_view): New concept.
>         (_Istream::operator()): Constrain it.  Move [[nodiscard]] to the
>         end of the function declarator.
>         * testsuite/std/ranges/iota/iota_view.cc (test07): New test.
>         * testsuite/std/ranges/istream_view.cc (test08): New test.
>         * testsuite/std/ranges/single_view.cc (test07): New test.
> ---
>  libstdc++-v3/include/std/ranges               | 40 ++++++++++++++-----
>  .../testsuite/std/ranges/iota/iota_view.cc    | 10 +++++
>  .../testsuite/std/ranges/istream_view.cc      | 12 ++++++
>  .../testsuite/std/ranges/single_view.cc       | 13 ++++++
>  4 files changed, 65 insertions(+), 10 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/ranges b/libstdc++-v3/include/std/ranges
> index 0a65d74bb5b..67566c6ebcf 100644
> --- a/libstdc++-v3/include/std/ranges
> +++ b/libstdc++-v3/include/std/ranges
> @@ -675,30 +675,41 @@ namespace views
>    template<typename _Tp>
>      inline constexpr empty_view<_Tp> empty{};
>
> +  namespace __detail
> +  {
> +    template<typename _Tp>
> +      concept __can_single_view
> +       = requires { single_view<decay_t<_Tp>>(std::declval<_Tp>()); };
> +  } // namespace __detail
> +
>    struct _Single
>    {
> -    template<typename _Tp>
> -      [[nodiscard]]
> +    template<__detail::__can_single_view _Tp>
>        constexpr auto
> -      operator()(_Tp&& __e) const
> +      operator() [[nodiscard]] (_Tp&& __e) const
>        noexcept(noexcept(single_view<decay_t<_Tp>>(std::forward<_Tp>(__e))))
>        { return single_view<decay_t<_Tp>>(std::forward<_Tp>(__e)); }
>    };
>
>    inline constexpr _Single single{};
>
> +  namespace __detail
> +  {
> +    template<typename... _Args>
> +      concept __can_iota_view = requires { iota_view(std::declval<_Args>()...); };
> +  } // namespace __detail
> +
>    struct _Iota
>    {
> -    template<typename _Tp>
> -      [[nodiscard]]
> +    template<__detail::__can_iota_view _Tp>
>        constexpr auto
> -      operator()(_Tp&& __e) const
> +      operator() [[nodiscard]] (_Tp&& __e) const
>        { return iota_view(std::forward<_Tp>(__e)); }
>
>      template<typename _Tp, typename _Up>
> -      [[nodiscard]]
> +      requires __detail::__can_iota_view<_Tp, _Up>
>        constexpr auto
> -      operator()(_Tp&& __e, _Up&& __f) const
> +      operator() [[nodiscard]] (_Tp&& __e, _Up&& __f) const
>        { return iota_view(std::forward<_Tp>(__e), std::forward<_Up>(__f)); }
>    };
>
> @@ -796,13 +807,22 @@ namespace views
>
>  namespace views
>  {
> +  namespace __detail
> +  {
> +    template<typename _Tp, typename _Up>
> +    concept __can_istream_view
> +      = requires (_Up __e) {
> +       basic_istream_view<_Tp, typename _Up::char_type, typename _Up::traits_type>(__e);
> +      };
> +  };
> +
>    template<typename _Tp>
>      struct _Istream
>      {
>        template<typename _CharT, typename _Traits>
> -       [[nodiscard]]
>         constexpr auto
> -       operator()(basic_istream<_CharT, _Traits>& __e) const
> +       operator() [[nodiscard]] (basic_istream<_CharT, _Traits>& __e) const
> +       requires __detail::__can_istream_view<_Tp, std::remove_reference_t<decltype(__e)>>
>         { return basic_istream_view<_Tp, _CharT, _Traits>(__e); }
>      };
>
> diff --git a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
> index 2dd17113536..0d2eaf1d0c2 100644
> --- a/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/iota/iota_view.cc
> @@ -110,6 +110,15 @@ test06()
>    VERIFY( std::ranges::equal(v3, w3) );
>  }
>
> +template<auto iota = std::views::iota>
> +void
> +test07()
> +{
> +  // Verify SFINAE behavior.
> +  static_assert(!requires { iota(nullptr); });
> +  static_assert(!requires { iota(nullptr, nullptr); });
> +}
> +
>  int
>  main()
>  {
> @@ -119,4 +128,5 @@ main()
>    test04();
>    test05();
>    test06();
> +  test07();
>  }
> diff --git a/libstdc++-v3/testsuite/std/ranges/istream_view.cc b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> index 26f109fdeaa..cc1c3e006b9 100644
> --- a/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/istream_view.cc
> @@ -115,6 +115,17 @@ test07()
>    VERIFY( sum == 10 );
>  }
>
> +template<class T, class U>
> +concept can_istream_view = requires (U u) { views::istream<T>(u); };
> +
> +void
> +test08()
> +{
> +  // Verify SFINAE behavior.
> +  struct S { };
> +  static_assert(!can_istream_view<S, std::istringstream>);
> +}
> +
>  int
>  main()
>  {
> @@ -125,4 +136,5 @@ main()
>    test05();
>    test06();
>    test07();
> +  test08();
>  }
> diff --git a/libstdc++-v3/testsuite/std/ranges/single_view.cc b/libstdc++-v3/testsuite/std/ranges/single_view.cc
> index 063caeb8785..38a3946ca43 100644
> --- a/libstdc++-v3/testsuite/std/ranges/single_view.cc
> +++ b/libstdc++-v3/testsuite/std/ranges/single_view.cc
> @@ -111,6 +111,18 @@ test06()
>    auto y = std::views::single(std::move(obj));
>  }
>
> +template<auto single = std::views::single>
> +void
> +test07()
> +{
> +  // Verify SFINAE behavior.
> +  struct uncopyable {
> +    uncopyable();
> +    uncopyable(const uncopyable&) = delete;
> +  };
> +  static_assert(!requires { single(uncopyable{}); });
> +}
> +
>  int main()
>  {
>    test01();
> @@ -119,4 +131,5 @@ int main()
>    test04();
>    test05();
>    test06();
> +  test07();
>  }
> --
> 2.40.0.rc0.57.g454dfcbddf
>