[gcc r15-3774] libstdc++: Fix formatting of most negative chrono::duration [PR116755]

[Jonathan Wakely via Gcc-cvs]

https://gcc.gnu.org/g:482e651f5750e4648ade90e32ed45b094538e7f8

commit r15-3774-g482e651f5750e4648ade90e32ed45b094538e7f8
Author: Jonathan Wakely <jwak...@redhat.com>
Date:   Wed Sep 18 17:20:29 2024 +0100

    libstdc++: Fix formatting of most negative chrono::duration [PR116755]
    
    When formatting chrono::duration<signed-integer-type, P>::min() we were
    causing undefined behaviour by trying to form the negative of the most
    negative value. If we convert negative durations with integer rep to the
    corresponding unsigned integer rep then we can safely represent all
    values.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/116755
            * include/bits/chrono_io.h (formatter<duration<R,P>>::format):
            Cast negative integral durations to unsigned rep.
            * testsuite/20_util/duration/io.cc: Test the most negative
            integer durations.

Diff:
---
 libstdc++-v3/include/bits/chrono_io.h         | 16 ++++++++++++++--
 libstdc++-v3/testsuite/20_util/duration/io.cc |  8 ++++++++
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h 
b/libstdc++-v3/include/bits/chrono_io.h
index 0e4d23c9bb77..c7d2c9862fcf 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -1720,8 +1720,20 @@ namespace __format
               basic_format_context<_Out, _CharT>& __fc) const
        {
          if constexpr (numeric_limits<_Rep>::is_signed)
-           if (__d < __d.zero())
-             return _M_f._M_format(-__d, __fc, true);
+           if (__d < __d.zero()) [[unlikely]]
+             {
+               if constexpr (is_integral_v<_Rep>)
+                 {
+                   // -d is undefined for the most negative integer.
+                   // Convert duration to corresponding unsigned rep.
+                   using _URep = make_unsigned_t<_Rep>;
+                   auto __ucnt = -static_cast<_URep>(__d.count());
+                   auto __ud = chrono::duration<_URep, _Period>(__ucnt);
+                   return _M_f._M_format(__ud, __fc, true);
+                 }
+               else
+                 return _M_f._M_format(-__d, __fc, true);
+             }
          return _M_f._M_format(__d, __fc, false);
        }
 
diff --git a/libstdc++-v3/testsuite/20_util/duration/io.cc 
b/libstdc++-v3/testsuite/20_util/duration/io.cc
index 6b00689672c8..57020f4f9537 100644
--- a/libstdc++-v3/testsuite/20_util/duration/io.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/io.cc
@@ -106,6 +106,14 @@ test_format()
   VERIFY( s == "500ms" );
   s = std::format("{:%Q %q}", u);
   VERIFY( s == "500 ms" );
+
+  // PR libstdc++/116755 extra minus sign for most negative value
+  auto minsec = std::chrono::seconds::min();
+  s = std::format("{}", minsec);
+  auto expected = std::format("{}s", minsec.count());
+  VERIFY( s == expected );
+  s = std::format("{:%Q%q}", minsec);
+  VERIFY( s == expected );
 }
 
 void