[gcc r16-441] libstdc++: Add header mdspan to the build-system.

[Jonathan Wakely]

https://gcc.gnu.org/g:4c9eef71db5e8e693c44ac4090247541e5a43063

commit r16-441-g4c9eef71db5e8e693c44ac4090247541e5a43063
Author: Luc Grosheintz <luc.grosheintz@gmail.com>
Date:   Tue Apr 29 14:46:08 2025 +0200

    libstdc++: Add header mdspan to the build-system.
    
    Creates a nearly empty header mdspan and adds it to the build-system and
    Doxygen config file.
    
    libstdc++-v3/ChangeLog:
    
            * doc/doxygen/user.cfg.in: Add <mdspan>.
            * include/Makefile.am: Ditto.
            * include/Makefile.in: Ditto.
            * include/precompiled/stdc++.h: Ditto.
            * include/std/mdspan: New file.
    
    Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

Diff:
---
 libstdc++-v3/doc/doxygen/user.cfg.in      |  1 +
 libstdc++-v3/include/Makefile.am          |  1 +
 libstdc++-v3/include/Makefile.in          |  1 +
 libstdc++-v3/include/precompiled/stdc++.h |  1 +
 libstdc++-v3/include/std/mdspan           | 48 +++++++++++++++++++++++++++++++
 5 files changed, 52 insertions(+)

diff --git a/libstdc++-v3/doc/doxygen/user.cfg.in b/libstdc++-v3/doc/doxygen/user.cfg.in
index 19ae67a67bac..e926c6707f67 100644
--- a/libstdc++-v3/doc/doxygen/user.cfg.in
+++ b/libstdc++-v3/doc/doxygen/user.cfg.in
@@ -880,6 +880,7 @@ INPUT                  = @srcdir@/doc/doxygen/doxygroups.cc \
                          include/list \
                          include/locale \
                          include/map \
+                         include/mdspan \
                          include/memory \
                          include/memory_resource \
                          include/mutex \
diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 537774c26689..1140fa0dffd1 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -38,6 +38,7 @@ std_freestanding = \
 	${std_srcdir}/generator \
 	${std_srcdir}/iterator \
 	${std_srcdir}/limits \
+	${std_srcdir}/mdspan \
 	${std_srcdir}/memory \
 	${std_srcdir}/numbers \
 	${std_srcdir}/numeric \
diff --git a/libstdc++-v3/include/Makefile.in b/libstdc++-v3/include/Makefile.in
index 7b96b2207f84..c96e981acd6e 100644
--- a/libstdc++-v3/include/Makefile.in
+++ b/libstdc++-v3/include/Makefile.in
@@ -396,6 +396,7 @@ std_freestanding = \
 	${std_srcdir}/generator \
 	${std_srcdir}/iterator \
 	${std_srcdir}/limits \
+	${std_srcdir}/mdspan \
 	${std_srcdir}/memory \
 	${std_srcdir}/numbers \
 	${std_srcdir}/numeric \
diff --git a/libstdc++-v3/include/precompiled/stdc++.h b/libstdc++-v3/include/precompiled/stdc++.h
index f4b312d9e470..e7d89c927049 100644
--- a/libstdc++-v3/include/precompiled/stdc++.h
+++ b/libstdc++-v3/include/precompiled/stdc++.h
@@ -228,6 +228,7 @@
 #include <flat_map>
 #include <flat_set>
 #include <generator>
+#include <mdspan>
 #include <print>
 #include <spanstream>
 #include <stacktrace>
diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
new file mode 100644
index 000000000000..78a00a5aa529
--- /dev/null
+++ b/libstdc++-v3/include/std/mdspan
@@ -0,0 +1,48 @@
+// <mdspan> -*- C++ -*-
+
+// Copyright The GNU Toolchain Authors.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file mdspan
+ *  This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_MDSPAN
+#define _GLIBCXX_MDSPAN 1
+
+#ifdef _GLIBCXX_SYSHDR
+#pragma GCC system_header
+#endif
+
+#define __glibcxx_want_mdspan
+#include <bits/version.h>
+
+#ifdef __glibcxx_mdspan
+
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
+_GLIBCXX_END_NAMESPACE_VERSION
+}
+#endif
+#endif