[gcc r14-8320] libstdc++: Fix std::chrono::file_clock conversions for low-precision times

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

https://gcc.gnu.org/g:fba15da8fa518bfb8d3e061795bc3ca2dea01d27

commit r14-8320-gfba15da8fa518bfb8d3e061795bc3ca2dea01d27
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sun Jan 21 18:11:16 2024 +0000

    libstdc++: Fix std::chrono::file_clock conversions for low-precision times
    
    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.

Diff:
---
 libstdc++-v3/include/bits/chrono.h                    | 14 ++++++++------
 libstdc++-v3/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();
 }