[gcc r12-3019] libstdc++: Improve overflow check for file timestamps

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:65441d8fc3c132a58c8bef6faefa2bc25e82a913

commit r12-3019-g65441d8fc3c132a58c8bef6faefa2bc25e82a913
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Aug 19 11:03:01 2021 +0100

    libstdc++: Improve overflow check for file timestamps
    
    The current code assumes that system_clock::duration is nanoseconds, and
    also performs a value-changing conversion from nanoseconds::max() to
    double (which doesn't matter after dividing by 1e9, but triggers a
    warning with Clang nonetheless).
    
    A better solution is to use system_clock::duration::max() and perform
    the comparison entirely using the std::chrono types, rather than with
    dimensionless arithmetic types.
    
    This doesn't address the FIXME in the function, so the overflow check
    still rejects some values that could be represented by the file_clock.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * src/filesystem/ops-common.h (filesystem::file_time): Improve
            overflow check by using system_clock::duration::max().

Diff:
---
 libstdc++-v3/src/filesystem/ops-common.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/src/filesystem/ops-common.h b/libstdc++-v3/src/filesystem/ops-common.h
index 304e5b263fb..bf26c06b7b5 100644
--- a/libstdc++-v3/src/filesystem/ops-common.h
+++ b/libstdc++-v3/src/filesystem/ops-common.h
@@ -229,7 +229,7 @@ namespace __gnu_posix
     // (This only applies to the C++17 Filesystem library, because for the
     // Filesystem TS we don't have a distinct __file_clock, we just use the
     // system clock for file timestamps).
-    if (s >= (nanoseconds::max().count() / 1e9))
+    if (seconds{s} >= floor<seconds>(system_clock::duration::max()))
       {
 	ec = std::make_error_code(std::errc::value_too_large); // EOVERFLOW
 	return system_clock::time_point::min();