[committed] libstdc++: Fix std::chrono::file_clock conversions for low-precision times

[Jonathan Wakely]

Tested aarch64-linux. Pushed to trunk. Backport needed to gcc-13 too.

-- >8 --

THe std::chrono::file_clock conversions were not using common_type and
so failed to compile when converting anything that should have increased
precision after arithmetic with a std::chrono::seconds value.

libstdc++-v3/ChangeLog:

        * include/bits/chrono.h (__file_clock::from_sys)
        (__file_clock::to_sys, __file_clock::_S_from_sys)
        (__file_clock::_S_to_sys): Use common_type for return type.
        * testsuite/std/time/clock/file/members.cc: Check round trip
        conversion for time with lower precision that seconds.
---
 libstdc++-v3/include/bits/chrono.h                 | 14 ++++++++------
 .../testsuite/std/time/clock/file/members.cc       |  9 +++++++++
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/libstdc++-v3/include/bits/chrono.h 
b/libstdc++-v3/include/bits/chrono.h
index 6f4fe55f9c0..0773867da71 100644
--- a/libstdc++-v3/include/bits/chrono.h
+++ b/libstdc++-v3/include/bits/chrono.h
@@ -1453,14 +1453,14 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
 #if __cplusplus > 201703L
       template<typename _Dur>
        static
-       chrono::file_time<_Dur>
+       chrono::file_time<common_type_t<_Dur, chrono::seconds>>
        from_sys(const chrono::sys_time<_Dur>& __t) noexcept
        { return _S_from_sys(__t); }
 
       // For internal use only
       template<typename _Dur>
        static
-       chrono::sys_time<_Dur>
+       chrono::sys_time<common_type_t<_Dur, chrono::seconds>>
        to_sys(const chrono::file_time<_Dur>& __t) noexcept
        { return _S_to_sys(__t); }
 #endif // C++20
@@ -1477,20 +1477,22 @@ _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
       // For internal use only
       template<typename _Dur>
        static
-       chrono::time_point<__file_clock, _Dur>
+       chrono::time_point<__file_clock, common_type_t<_Dur, chrono::seconds>>
        _S_from_sys(const chrono::time_point<__sys_clock, _Dur>& __t) noexcept
        {
-         using __file_time = chrono::time_point<__file_clock, _Dur>;
+         using _CDur = common_type_t<_Dur, chrono::seconds>;
+         using __file_time = chrono::time_point<__file_clock, _CDur>;
          return __file_time{__t.time_since_epoch()} - _S_epoch_diff;
        }
 
       // For internal use only
       template<typename _Dur>
        static
-       chrono::time_point<__sys_clock, _Dur>
+       chrono::time_point<__sys_clock, common_type_t<_Dur, chrono::seconds>>
        _S_to_sys(const chrono::time_point<__file_clock, _Dur>& __t) noexcept
        {
-         using __sys_time = chrono::time_point<__sys_clock, _Dur>;
+         using _CDur = common_type_t<_Dur, chrono::seconds>;
+         using __sys_time = chrono::time_point<__sys_clock, _CDur>;
          return __sys_time{__t.time_since_epoch()} + _S_epoch_diff;
        }
     };
diff --git a/libstdc++-v3/testsuite/std/time/clock/file/members.cc 
b/libstdc++-v3/testsuite/std/time/clock/file/members.cc
index 54459cc56a9..9d217b17811 100644
--- a/libstdc++-v3/testsuite/std/time/clock/file/members.cc
+++ b/libstdc++-v3/testsuite/std/time/clock/file/members.cc
@@ -41,9 +41,18 @@ test02()
   VERIFY( t - s < 1s );
 }
 
+void
+test03()
+{
+  using namespace std::chrono;
+  auto st = sys_days(2024y/January/21);
+  VERIFY( file_clock::to_sys(file_clock::from_sys(st)) == st );
+}
+
 int
 main()
 {
   test01();
   test02();
+  test03();
 }
-- 
2.43.0