On Mon, 21 Jul 2025, 12:08 Luc Grosheintz, <luc.groshei...@gmail.com> wrote:
> The mdspan::is_{,always}_{unique,strided,exhaustive} methods only call
> their counterparts in mdspan::mapping_type. The standard specifies that
> the methods of mdspan::mapping_type are noexcept, but doesn't specify if
> the methods of mdspan are noexcept.
>
> Libc++ strengthened the exception guarantee for these mdspan methods.
> This commit conditionally strengthens these methods for libstdc++.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/mdspan (mdspan::is_always_unique): Make
> conditionally noexcept.
> (mdspan::is_always_exhaustive): Ditto.
> (mdspan::is_always_strided): Ditto.
> (mdspan::is_unique): Ditto.
> (mdspan::is_exhaustive): Ditto.
> (mdspan::is_strided): Ditto.
> * testsuite/23_containers/mdspan/layout_like.h: Make noexcept
> configurable. Add ThrowingLayout.
> * testsuite/23_containers/mdspan/mdspan.cc: Add tests for
> noexcept.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/std/mdspan | 21 ++-
> .../23_containers/mdspan/layout_like.h | 134 +++++++++---------
> .../testsuite/23_containers/mdspan/mdspan.cc | 17 +++
> 3 files changed, 101 insertions(+), 71 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/mdspan
> b/libstdc++-v3/include/std/mdspan
> index 271fdb5d8c7..f71141f43a9 100644
> --- a/libstdc++-v3/include/std/mdspan
> +++ b/libstdc++-v3/include/std/mdspan
> @@ -1275,23 +1275,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> constexpr const accessor_type&
> accessor() const noexcept { return _M_accessor; }
>
> + // Strengthened noexcept for all `is_*` methods.
>
I have a slight preference for a blank like after this comment, because it
applies to several following members, not just the first one. Without the
blank like it looks like it's just a comment on the first one. Of course,
if you actually read the comment you realise it applies to them all :-)
OK for trunk anyway.
static constexpr bool
> - is_always_unique() { return mapping_type::is_always_unique(); }
> + is_always_unique()
> noexcept(noexcept(mapping_type::is_always_unique()))
> + { return mapping_type::is_always_unique(); }
>
> static constexpr bool
> - is_always_exhaustive() { return
> mapping_type::is_always_exhaustive(); }
> + is_always_exhaustive()
> + noexcept(noexcept(mapping_type::is_always_exhaustive()))
> + { return mapping_type::is_always_exhaustive(); }
>
> static constexpr bool
> - is_always_strided() { return mapping_type::is_always_strided(); }
> + is_always_strided()
> + noexcept(noexcept(mapping_type::is_always_strided()))
> + { return mapping_type::is_always_strided(); }
>
> constexpr bool
> - is_unique() const { return _M_mapping.is_unique(); }
> + is_unique() const noexcept(noexcept(_M_mapping.is_unique()))
> + { return _M_mapping.is_unique(); }
>
> constexpr bool
> - is_exhaustive() const { return _M_mapping.is_exhaustive(); }
> + is_exhaustive() const noexcept(noexcept(_M_mapping.is_exhaustive()))
> + { return _M_mapping.is_exhaustive(); }
>
> constexpr bool
> - is_strided() const { return _M_mapping. is_strided(); }
> + is_strided() const noexcept(noexcept(_M_mapping.is_strided()))
> + { return _M_mapping. is_strided(); }
>
> constexpr index_type
> stride(rank_type __r) const { return _M_mapping.stride(__r); }
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/layout_like.h
> b/libstdc++-v3/testsuite/23_containers/mdspan/layout_like.h
> index 6a0f8cafaa7..ded6e9d0cc9 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/layout_like.h
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/layout_like.h
> @@ -1,83 +1,87 @@
> #ifndef TEST_MDSPAN_LAYOUT_LIKE_H
> #define TEST_MDSPAN_LAYOUT_LIKE_H 1
>
> -struct LayoutLike
> -{
> - template<typename Extents>
> - class mapping
> - {
> - public:
> - using extents_type = Extents;
> - using index_type = typename extents_type::index_type;
> - using size_type = typename extents_type::size_type;
> - using rank_type = typename extents_type::rank_type;
> - using layout_type = LayoutLike;
> -
> - constexpr
> - mapping() noexcept = default;
> -
> - constexpr
> - mapping(Extents exts)
> - : m_exts(exts)
> - { }
> -
> - constexpr const extents_type&
> - extents() const noexcept { return m_exts; }
> -
> - constexpr index_type
> - required_span_size() const noexcept
> +template<bool Noexcept>
> + struct CustomLayout
> + {
> + template<typename Extents>
> + class mapping
> {
> - for (size_t i = 0; i < extents_type::rank(); ++i)
> - if (m_exts.extent(i) == 0)
> - return 0;
> - return 1;
> - }
> -
> - template<typename... Indices>
> - requires (sizeof...(Indices) == extents_type::rank())
> + public:
> + using extents_type = Extents;
> + using index_type = typename extents_type::index_type;
> + using size_type = typename extents_type::size_type;
> + using rank_type = typename extents_type::rank_type;
> + using layout_type = CustomLayout;
> +
> + constexpr
> + mapping() noexcept = default;
> +
> + constexpr
> + mapping(Extents exts)
> + : m_exts(exts)
> + { }
> +
> + constexpr const extents_type&
> + extents() const noexcept(Noexcept) { return m_exts; }
> +
> constexpr index_type
> - operator()(Indices...) const noexcept
> + required_span_size() const noexcept(Noexcept)
> + {
> + for (size_t i = 0; i < extents_type::rank(); ++i)
> + if (m_exts.extent(i) == 0)
> + return 0;
> + return 1;
> + }
> +
> + template<typename... Indices>
> + requires (sizeof...(Indices) == extents_type::rank())
> + constexpr index_type
> + operator()(Indices...) const noexcept(Noexcept)
> + { return 0; }
> +
> + static constexpr index_type
> + stride(rank_type) noexcept(Noexcept)
> { return 0; }
>
> - static constexpr index_type
> - stride(rank_type) noexcept
> - { return 0; }
> + static constexpr bool
> + is_always_unique() noexcept(Noexcept)
> + { return false; }
>
> - static constexpr bool
> - is_always_unique() noexcept
> - { return false; }
> + static constexpr bool
> + is_always_exhaustive() noexcept(Noexcept)
> + { return true; }
>
> - static constexpr bool
> - is_always_exhaustive() noexcept
> - { return true; }
> + static constexpr bool
> + is_always_strided() noexcept(Noexcept)
> + { return true; }
>
> - static constexpr bool
> - is_always_strided() noexcept
> - { return true; }
> + constexpr bool
> + is_unique() const noexcept(Noexcept)
> + {
> + if (required_span_size() == 0)
> + return true;
>
> - constexpr bool
> - is_unique() noexcept
> - {
> - if (required_span_size() == 0)
> + for (size_t i = 0; i < extents_type::rank(); ++i)
> + if (m_exts.extent(i) > 1)
> + return false;
> return true;
> + }
>
> - for (size_t i = 0; i < extents_type::rank(); ++i)
> - if (m_exts.extent(i) > 1)
> - return false;
> - return true;
> - }
> + static constexpr bool
> + is_exhaustive() noexcept(Noexcept)
> + { return true; }
>
> - static constexpr bool
> - is_exhaustive() noexcept
> - { return true; }
> + static constexpr bool
> + is_strided() noexcept(Noexcept)
> + { return true; }
>
> - static constexpr bool
> - is_strided() noexcept
> - { return true; }
> + private:
> + Extents m_exts;
> + };
> + };
>
> - private:
> - Extents m_exts;
> - };
> -};
> +using LayoutLike = CustomLayout<true>;
> +using ThrowingLayout = CustomLayout<false>;
>
> #endif
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> index be4a1b1c17e..942f6d96dca 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> @@ -650,6 +650,21 @@ test_nothrow_movable_all()
> test_nothrow_movable<false, false>();
> }
>
> +template<typename Layout, bool Expected>
> + constexpr void
> + test_nothrow_is_methods()
> + {
> + using Extents = std::extents<int, dyn>;
> + using MDSpan = std::mdspan<double, Extents, Layout>;
> + static_assert(noexcept(MDSpan::is_always_unique()) == Expected);
> + static_assert(noexcept(MDSpan::is_always_exhaustive()) == Expected);
> + static_assert(noexcept(MDSpan::is_always_strided()) == Expected);
> +
> + static_assert(noexcept(std::declval<MDSpan>().is_unique()) ==
> Expected);
> + static_assert(noexcept(std::declval<MDSpan>().is_exhaustive()) ==
> Expected);
> + static_assert(noexcept(std::declval<MDSpan>().is_strided()) ==
> Expected);
> + }
> +
> int
> main()
> {
> @@ -713,5 +728,7 @@ main()
> test_swap_adl();
>
> test_nothrow_movable_all();
> + test_nothrow_is_methods<std::layout_right, true>();
> + test_nothrow_is_methods<ThrowingLayout, false>();
> return 0;
> }
> --
> 2.50.0
>
>
