From dfab6593b05a65b5a475e0572e101bd56e3a2282 Mon Sep 17 00:00:00 2001
From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Mon, 21 Jul 2025 13:07:37 +0200
Subject: [PATCH] libstdc++: Make the default ctor of mdspan conditionally
 noexcept.

Previously, the default ctor of mdspan was never noexcept, even if all
members of mdspan were nothrow default constructible.

This commit makes mdspan conditionally nothrow default constructible.
A similar strengthening happens in libc++.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (mdspan::mdspan): Make default ctor
	conditionally noexcept.
	* testsuite/23_containers/mdspan/mdspan.cc: Add tests.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
---
 libstdc++-v3/include/std/mdspan               | 12 +++---
 .../testsuite/23_containers/mdspan/mdspan.cc  | 37 +++++++++++++++++++
 2 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index 89b9f3c8aba..055778d2968 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -1114,11 +1114,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       constexpr
       mdspan()
       requires (rank_dynamic() > 0)
-          && is_default_constructible_v<data_handle_type>
+	  && is_default_constructible_v<data_handle_type>
 	  && is_default_constructible_v<mapping_type>
-	  && is_default_constructible_v<accessor_type>
-      : _M_accessor(), _M_mapping(), _M_handle()
-      { }
+	  && is_default_constructible_v<accessor_type> = default;
 
       constexpr
       mdspan(const mdspan& __other) = default;
@@ -1307,9 +1305,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       stride(rank_type __r) const { return _M_mapping.stride(__r); }
 
     private:
-      [[no_unique_address]] accessor_type _M_accessor;
-      [[no_unique_address]] mapping_type _M_mapping;
-      [[no_unique_address]] data_handle_type _M_handle;
+      [[no_unique_address]] accessor_type _M_accessor = accessor_type();
+      [[no_unique_address]] mapping_type _M_mapping = mapping_type();
+      [[no_unique_address]] data_handle_type _M_handle = data_handle_type();
     };
 
   template<typename _CArray>
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
index 942f6d96dca..6ffddd11e95 100644
--- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
@@ -4,6 +4,7 @@
 #include <testsuite_hooks.h>
 #include "int_like.h"
 #include "layout_like.h"
+#include <stdexcept>
 
 constexpr auto dyn = std::dynamic_extent;
 
@@ -114,6 +115,27 @@ test_class_properties_all()
   return true;
 }
 
+template<typename T>
+  class ThrowingDefaultAccessor
+  {
+  public:
+    using element_type = T;
+    using reference = element_type&;
+    using data_handle_type = element_type*;
+    using offset_policy = ThrowingDefaultAccessor;
+
+    ThrowingDefaultAccessor() noexcept(false)
+    { }
+
+    reference
+    access(data_handle_type p, size_t i) const
+    { return p[i]; }
+
+    typename offset_policy::data_handle_type
+    offset(data_handle_type p, size_t i) const
+    { return p + i; }
+  };
+
 constexpr bool
 test_default_ctor()
 {
@@ -130,6 +152,18 @@ test_default_ctor()
   return true;
 }
 
+template<template<typename T> typename Accessor, bool Expected>
+  constexpr void
+  test_nothrow_default_ctor()
+  {
+    using Extents = std::extents<int, dyn>;
+    using Layout = std::layout_left;
+    using MDSpan = std::mdspan<double, Extents, Layout, Accessor<double>>;
+
+    static_assert(std::is_default_constructible_v<MDSpan>);
+    static_assert(std::is_nothrow_default_constructible_v<MDSpan> == Expected);
+  }
+
 constexpr bool
 test_from_other()
 {
@@ -683,6 +717,9 @@ main()
   test_default_ctor();
   static_assert(test_default_ctor());
 
+  test_nothrow_default_ctor<std::default_accessor, true>();
+  test_nothrow_default_ctor<ThrowingDefaultAccessor, false>();
+
   test_from_other();
   static_assert(test_from_other());
 
-- 
2.47.2