[PATCH v4 8/8] libstdc++: Make layout_left(layout_stride) noexcept.

[Luc Grosheintz]

[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 implementations of the STL make the same choice.

libstdc++-v3/ChangeLog:

        * include/std/mdspan: 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.groshei...@gmail.com>
---
 libstdc++-v3/include/std/mdspan               |  6 +++-
 .../23_containers/mdspan/layouts/ctors.cc     | 33 ++++++++-----------
 2 files changed, 19 insertions(+), 20 deletions(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 33ad5070a37..2a7f0452cd7 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -561,10 +561,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
        : mapping(__other.extents(), __mdspan::__internal_ctor{})
        { }
 
+      // [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.
       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 2507eeaf7a1..6f813e9877b 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/layouts/ctors.cc
@@ -362,30 +362,24 @@ 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>
@@ -404,31 +398,32 @@ 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;
     }
-- 
2.49.0