[PATCH v1 1/4] libstdc++: Setup internal FTM for mdspan.

[Luc Grosheintz]

Uses the FTM infrastructure to create an internal feature testing macro
for partial availability of mdspan; which is then used to hide the
contents of the header mdspan when compiling against a standard prior to
C++23.

libstdc++-v3/ChangeLog:

        * include/bits/version.def: Add internal feature testing macro
        __glibcxx_mdspan.
        * include/bits/version.h: Regenerate.

Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
---
 libstdc++-v3/include/bits/version.def |  9 +++++++++
 libstdc++-v3/include/bits/version.h   | 10 ++++++++++
 2 files changed, 19 insertions(+)

diff --git a/libstdc++-v3/include/bits/version.def 
b/libstdc++-v3/include/bits/version.def
index 8f609b469cc..89f066010ae 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -998,6 +998,15 @@ ftms = {
   };
 };
 
+ftms = {
+  name = mdspan;
+  no_stdname = true; // FIXME: remove
+  values = {
+    v = 1; // FIXME: 202207
+    cxxmin = 23;
+  };
+};
+
 ftms = {
   name = ssize;
   values = {
diff --git a/libstdc++-v3/include/bits/version.h 
b/libstdc++-v3/include/bits/version.h
index f05c3fd13c0..b97053a891e 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -1115,6 +1115,16 @@
 #endif /* !defined(__cpp_lib_span) && defined(__glibcxx_want_span) */
 #undef __glibcxx_want_span
 
+#if !defined(__cpp_lib_mdspan)
+# if (__cplusplus >= 202100L)
+#  define __glibcxx_mdspan 1L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_mdspan)
+#   define __cpp_lib_mdspan 1L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_mdspan) && defined(__glibcxx_want_mdspan) */
+#undef __glibcxx_want_mdspan
+
 #if !defined(__cpp_lib_ssize)
 # if (__cplusplus >= 202002L)
 #  define __glibcxx_ssize 201902L
-- 
2.49.0