[gcc r17-2857] libstdc++: Implement LWG 4440 feature-test macros added to <iosfwd>

https://gcc.gnu.org/g:0cb732698c1cf5b5d7c4e2da64ad506e55fae551

commit r17-2857-g0cb732698c1cf5b5d7c4e2da64ad506e55fae551
Author: Patrick Palka <[email protected]>
Date:   Fri Jul 31 14:11:42 2026 -0400

    libstdc++: Implement LWG 4440 feature-test macros added to <iosfwd>
    
    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.
    
    Reviewed-by: Tomasz Kamiński <[email protected]>
    Reviewed-by: Jonathan Wakely <[email protected]>

Diff:
---
 libstdc++-v3/include/std/iosfwd                         |  4 ++++
 libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc | 12 ++++++++++++
 2 files changed, 16 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..81b158b81285 100644
--- a/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
+++ b/libstdc++-v3/testsuite/27_io/headers/iosfwd/synopsis.cc
@@ -3,6 +3,18 @@
 
 #include <iosfwd>
 
+#if __cplusplus >= 202002L && __cpp_lib_char8_t != 201907L
+# error "Feature-test macro __cpp_lib_char8_t has wrong value in <iosfwd>"
+#endif
+
+#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]