[gcc r16-1473] libstdc++: Make layout_left(layout_stride) noexcept.

[Tomasz Kaminski]

https://gcc.gnu.org/g:1620e1ac8f5654678245f9f0eda876d0dec0b567

commit r16-1473-g1620e1ac8f5654678245f9f0eda876d0dec0b567
Author: Luc Grosheintz <luc.grosheintz@gmail.com>
Date:   Wed Jun 4 16:58:53 2025 +0200

    libstdc++: Make layout_left(layout_stride) noexcept.
    
    [mdspan.layout.left.cons] of N4950 states that this ctor is not
    noexcept. Since, all other ctors of layout_left, layout_right or
    layout_stride are noexcept, the choice was made, based on
    [res.on.exception.handling], to make this ctor noexcept.
    
    Two other major standard library implementations make the same choice.
    
    libstdc++-v3/ChangeLog:
    
            * include/std/mdspan (layout_left): Strengthen the exception
            guarantees of layout_left::mapping(layout_stride::mapping).
            * testsuite/23_containers/mdspan/layouts/ctors.cc:
            Simplify tests to reflect the change.
    
    Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

Diff:
---
 libstdc++-v3/include/std/mdspan                    |  3 +-
 .../23_containers/mdspan/layouts/ctors.cc          | 33 +++++++++-------------
 2 files changed, 16 insertions(+), 20 deletions(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index fe182c35a558..4a3e863bed5f 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -561,10 +561,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	: mapping(__other.extents(), __mdspan::__internal_ctor{})
 	{ }
 
+      // noexcept for consistency with other layouts.
       template<typename _OExtents>
 	requires is_constructible_v<extents_type, _OExtents>
 	constexpr explicit(extents_type::rank() > 0)
-	mapping(const layout_stride::mapping<_OExtents>& __other)
+	mapping(const layout_stride::mapping<_OExtents>& __other) noexcept
 	: mapping(__other.extents(), __mdspan::__internal_ctor{})
 	{ __glibcxx_assert(*this == __other); }
 
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
index 92507a8e769d..23c0a55dae19 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
@@ -339,30 +339,24 @@ namespace from_stride
 
   template<typename Layout, typename Extents, typename OExtents>
     constexpr void
-    verify_convertible(OExtents oexts)
+    verify_nothrow_convertible(OExtents oexts)
     {
       using Mapping = typename Layout::mapping<Extents>;
       using OMapping = std::layout_stride::mapping<OExtents>;
 
       constexpr auto other = OMapping(oexts, strides(Mapping(Extents(oexts))));
-      if constexpr (std::is_same_v<Layout, std::layout_right>)
-	::verify_nothrow_convertible<Mapping>(other);
-      else
-	::verify_convertible<Mapping>(other);
+      ::verify_nothrow_convertible<Mapping>(other);
     }
 
   template<typename Layout, typename Extents, typename OExtents>
     constexpr void
-    verify_constructible(OExtents oexts)
+    verify_nothrow_constructible(OExtents oexts)
     {
       using Mapping = typename Layout::mapping<Extents>;
       using OMapping = std::layout_stride::mapping<OExtents>;
 
       constexpr auto other = OMapping(oexts, strides(Mapping(Extents(oexts))));
-      if constexpr (std::is_same_v<Layout, std::layout_right>)
-	::verify_nothrow_constructible<Mapping>(other);
-      else
-	::verify_constructible<Mapping>(other);
+      ::verify_nothrow_constructible<Mapping>(other);
     }
 
   template<typename Layout>
@@ -381,31 +375,32 @@ namespace from_stride
 	typename Layout::mapping<std::extents<int, 2>>,
 	std::layout_stride::mapping<std::extents<int, 1>>>();
 
-      verify_convertible<Layout, std::extents<int>>(std::extents<int>{});
+      verify_nothrow_convertible<Layout, std::extents<int>>(
+	std::extents<int>{});
 
-      verify_convertible<Layout, std::extents<unsigned int>>(
+      verify_nothrow_convertible<Layout, std::extents<unsigned int>>(
 	std::extents<int>{});
 
       // Rank ==  0 doesn't check IndexType for convertibility.
-      verify_convertible<Layout, std::extents<int>>(
+      verify_nothrow_convertible<Layout, std::extents<int>>(
 	std::extents<unsigned int>{});
 
-      verify_constructible<Layout, std::extents<int, 3>>(
+      verify_nothrow_constructible<Layout, std::extents<int, 3>>(
 	std::extents<int, 3>{});
 
-      verify_constructible<Layout, std::extents<unsigned int, 3>>(
+      verify_nothrow_constructible<Layout, std::extents<unsigned int, 3>>(
 	std::extents<int, 3>{});
 
-      verify_constructible<Layout, std::extents<int, 3>>(
+      verify_nothrow_constructible<Layout, std::extents<int, 3>>(
 	std::extents<unsigned int, 3>{});
 
-      verify_constructible<Layout, std::extents<int, 3, 5>>(
+      verify_nothrow_constructible<Layout, std::extents<int, 3, 5>>(
 	std::extents<int, 3, 5>{});
 
-      verify_constructible<Layout, std::extents<unsigned int, 3, 5>>(
+      verify_nothrow_constructible<Layout, std::extents<unsigned int, 3, 5>>(
 	std::extents<int, 3, 5>{});
 
-      verify_constructible<Layout, std::extents<int, 3, 5>>(
+      verify_nothrow_constructible<Layout, std::extents<int, 3, 5>>(
 	std::extents<unsigned int, 3, 5>{});
       return true;
     }