[Bug libstdc++/99832] std::chrono::system_clock::to_time_t needs ABI tag for 32-bit time_t

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99832

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Maybe something like this:

diff --git a/libstdc++-v3/config/os/gnu-linux/os_defines.h
b/libstdc++-v3/config/os/gnu-linux/os_defines.h
index 0af29325388..f7c73560831 100644
--- a/libstdc++-v3/config/os/gnu-linux/os_defines.h
+++ b/libstdc++-v3/config/os/gnu-linux/os_defines.h
@@ -84,7 +84,13 @@
 // Since glibc 2.34 all pthreads functions are usable without linking to
 // libpthread.
 #  define _GLIBCXX_GTHREAD_USE_WEAK 0
-# endif
+// Since glibc 2.34 using -D_TIME_BITS=64 will enable 64-bit time_t
+// for "legacy ABIs", i.e. ones that historically used 32-bit time_t.
+// This internal glibc macro will be defined iff new 64-bit time_t is in use.
+#  ifdef __USE_TIME_BITS64
+#   define _GLIBCXX_TIME_BITS64 1
+#  endif
+# endif // glibc 2.34
 #endif // __linux__

 #endif
diff --git a/libstdc++-v3/include/bits/chrono.h
b/libstdc++-v3/include/bits/chrono.h
index 579c5a266be..a63782b92ff 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -1242,6 +1242,9 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
       now() noexcept;

       // Map to C API
+#ifdef _GLIBCXX_TIME_BITS64
+      [[__gnu__::__abi_tag__("__time64")]]
+#endif
       static std::time_t
       to_time_t(const time_point& __t) noexcept
       {
@@ -1249,6 +1252,9 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
                           (__t.time_since_epoch()).count());
       }

+#ifdef _GLIBCXX_TIME_BITS64
+      [[__gnu__::__abi_tag__("__time64")]]
+#endif
       static time_point
       from_time_t(std::time_t __t) noexcept
       {

Alternatively, in <bits/c++config.h> do:

#define _GLIBCXX_TIME_BITS64_ABI_TAG

and then in config/os/gnu-linux/os_defines.h:


#  ifdef __USE_TIME_BITS64
#   undef _GLIBCXX_TIME_BITS64_ABI_TAG
#   define _GLIBCXX_TIME_BITS64_ABI_TAG [[__gnu__::__abi_tag__("__time64")]]
#  endif

Then the chrono code can just use that unconditionally instead of using #ifdef

I think for musl, newer versions use 64-bit time_t unconditionally. I'm not
sure if we can (or need to) use the abi_tag there.