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

[Patrick Palka]

On Fri, 31 Jul 2026, Tomasz Kaminski wrote:

> 
> 
> On Fri, Jul 31, 2026 at 3:14 PM Patrick Palka <ppalka@redhat.com> wrote:
>       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
> 
> I would test that __cpp_lib_char8_t is defined when you include this header, even
> if it is for all headers.

Sure:

-- >8 --

Subject: [PATCH] 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.
---
 libstdc++-v3/include/std/iosfwd                      |  4 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2

Subject: [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 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2

Subject: [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 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2

Subject: [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 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2

Subject: [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 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2

Subject: [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 ++++
 .../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]
-- 
2.55.0.481.ga97fcc37c2