[PATCH] libstdc++: Implement LWG 4440 feature-test macros added to <iosfwd>

Tested on x86_64-pc-linux-gnu, does this look OK for trunk and
backports?

-- >8 --

Note __cpp_lib_char8_t doesn't use the <bits/version.h> mechanism, it's
defined in c++config and thus provided by all headers by default.

libstdc++-v3/ChangeLog:

	* include/std/iosfwd: Provide feature-test macros
	__cpp_lib_spanstream and __cpp_lib_syncbuf.
	* testsuite/27_io/headers/iosfwd/synopsis.cc: Verify values
	of these FTMs.
---
 libstdc++-v3/include/std/iosfwd                         | 4 ++++
 libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc | 8 ++++++++
 2 files changed, 12 insertions(+)

diff --git a/libstdc++-v3/include/std/iosfwd b/libstdc++-v3/include/std/iosfwd
index 1d5e684b53bb..511b2f3a23f1 100644
--- a/libstdc++-v3/include/std/iosfwd
+++ b/libstdc++-v3/include/std/iosfwd
@@ -39,6 +39,10 @@
 
 #include <bits/requires_hosted.h> // iostreams
 
+#define __glibcxx_want_spanstream
+#define __glibcxx_want_syncbuf
+#include <bits/version.h>
+
   /**
    *  @defgroup io I/O
    *
diff --git a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
index 8d9501f42ef4..f39a0b95a3d6 100644
--- a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
+++ b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
@@ -3,6 +3,14 @@
 
 #include <iosfwd>
 
+#if __cplusplus >= 202202L && __cpp_lib_spanstream != 202106L
+# error "Feature-test macro __cpp_lib_spanstrean has wrong value in <iosfwd>"
+#endif
+
+#if __cplusplus >= 202002L && __cpp_lib_syncbuf != 201803L
+# error "Feature-test macro __cpp_lib_syncbuf has wrong value in <iosfwd>"
+#endif
+
 namespace std
 {
   // [iosfwd.syn]
-- 
2.55.0.481.ga97fcc37c2