[PATCH v2] libstdc++: fix __cpp_lib_chrono for old-ABI-only builds

[hugo]

From: Hugo Beauzée-Luyssen <hugo.beau...@datadoghq.com>

__cpp_lib_chrono only reaches the C++20 threshold when
_GLIBCXX_USE_CXX11_ABI is set. But chrono::tzdb and the rest of the
C++20 calendar/timezone API are also declared when
_GLIBCXX_USE_DUAL_ABI is unset (old-ABI-only builds), the same
condition already used throughout <chrono> and <bits/chrono_io.h>.

Add a tier for that case in version.def, defining the macro to
201803 (distinct from the full 201907 new-ABI value) so it reflects
what's actually declared. <chrono>'s own include guard for the
headers tzdb needs can then go back to the plain __cpp_lib_chrono
check.

libstdc++-v3/ChangeLog:

        * include/bits/version.def (chrono): Add a 201803 tier for
        old-ABI-only builds.
        * include/bits/version.h: Regenerate.
        * include/std/chrono: Simplify include guard now that
        __cpp_lib_chrono accounts for old-ABI-only builds.

Assisted-by: Claude (claude-sonnet-5)
Signed-off-by: Hugo Beauzée-Luyssen <hugo.beau...@datadoghq.com>
---
 libstdc++-v3/include/bits/version.def | 6 ++++++
 libstdc++-v3/include/bits/version.h   | 5 +++++
 libstdc++-v3/include/std/chrono       | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/version.def 
b/libstdc++-v3/include/bits/version.def
index 9c523581c65..6db0d22b788 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -624,6 +624,12 @@ ftms = {
     hosted = yes;
     cxx11abi = yes; // std::chrono::tzdb requires cxx11 std::string
   };
+  values = {
+    v = 201803;
+    cxxmin = 20;
+    hosted = yes;
+    extra_cond = "!_GLIBCXX_USE_DUAL_ABI";
+  };
   values = {
     v = 201611;
     cxxmin = 17;
diff --git a/libstdc++-v3/include/bits/version.h 
b/libstdc++-v3/include/bits/version.h
index e0003054af6..07af61af056 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -691,6 +691,11 @@
 #  if defined(__glibcxx_want_all) || defined(__glibcxx_want_chrono)
 #   define __cpp_lib_chrono 201907L
 #  endif
+# elif (__cplusplus >= 202002L) && _GLIBCXX_HOSTED && (!_GLIBCXX_USE_DUAL_ABI)
+#  define __glibcxx_chrono 201803L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_chrono)
+#   define __cpp_lib_chrono 201803L
+#  endif
 # elif (__cplusplus >= 201703L) && _GLIBCXX_HOSTED
 #  define __glibcxx_chrono 201611L
 #  if defined(__glibcxx_want_all) || defined(__glibcxx_want_chrono)
diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 4116f1c815e..ce0902522d7 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -57,7 +57,7 @@
 # include <bits/stl_algo.h>     // upper_bound
 # include <bits/range_access.h> // begin/end for arrays
 #endif
-#if __cpp_lib_chrono >= 201803L // C++20 && HOSTED && USE_CXX11_ABI
+#if __cpp_lib_chrono >= 201803L
 # include <sstream>
 # include <string>
 # include <vector>
-- 
2.43.0