[committed] libstdc++: Fix %S format of duration with floating-point rep

[Jonathan Wakely]

Tested x86_64-linux. Pushed to trunk. Backport needed too.

-- >8 --

I got the order of arguments to std::format_to wrong. It was in a
discarded statement, for a case which wasn't being tested.

libstdc++-v3/ChangeLog:

        * include/bits/chrono_io.h (__formatter_chrono::_M_S): Fix order
        of arguments to std::format_to.
        * testsuite/20_util/duration/io.cc: Test subsecond duration with
        floating-point rep.
---
 libstdc++-v3/include/bits/chrono_io.h         | 2 +-
 libstdc++-v3/testsuite/20_util/duration/io.cc | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/chrono_io.h 
b/libstdc++-v3/include/bits/chrono_io.h
index bcd76e4ab7b..c30451651ea 100644
--- a/libstdc++-v3/include/bits/chrono_io.h
+++ b/libstdc++-v3/include/bits/chrono_io.h
@@ -1137,7 +1137,7 @@ namespace __format
              using rep = typename decltype(__ss)::rep;
              if constexpr (is_floating_point_v<rep>)
                {
-                 __out = std::format_to(__loc, std::move(__out),
+                 __out = std::format_to(std::move(__out), __loc,
                                         _GLIBCXX_WIDEN("{:.{}Lg}"),
                                         __ss.count(),
                                         __hms.fractional_width);
diff --git a/libstdc++-v3/testsuite/20_util/duration/io.cc 
b/libstdc++-v3/testsuite/20_util/duration/io.cc
index d2b9de73f83..0582c0075a5 100644
--- a/libstdc++-v3/testsuite/20_util/duration/io.cc
+++ b/libstdc++-v3/testsuite/20_util/duration/io.cc
@@ -95,6 +95,10 @@ test_format()
                    "required by the chrono-specs") != s.npos);
     }
   }
+
+  std::chrono::duration<float, std::milli> d{0.5};
+  s = std::format("{}", d);
+  VERIFY( s == "0.5ms" );
 }
 
 void
-- 
2.43.0