<chrono: Rewrite %r spec for C locale (#1865)

put_time(%r) does the wrong thing when we use the C locale, due to some
internal machinery. It could get fixed further down level, but that
change is a lot more impactful. Instead, we simply rewrite %r when the C
locale is used in chrono.

This basically has to do with the way that _Strftime, _Gettnames, and
expand_time work together. _Gettnames returns a copy of its data and
expand_time figures out the locale based on pointer comparison.

stl/inc/chrono

@@ -6014,6 +6014,20 @@ namespace chrono {
                 }
                 _Os << _Time.tm_mday;
                 return true;
+            case 'r':
+                if constexpr (_Is_specialization_v<_Ty, hh_mm_ss>) {
+                    // put_time uses _Strftime in order to bypass reference-counting that locale uses. This function
+                    // takes the locale information by pointer, but the pointer (from _Gettnames) returns a copy.
+                    // _Strftime delegates to other functions but eventually (for the C locale) has the %r specifier
+                    // rewritten. It checks for the locale by comparing pointers, which do not compare equal as we have
+                    // a copy of the pointer instead of the original. Therefore, we replace %r for the C locale
+                    // ourselves.
+                    if (_Os.getloc() == locale::classic()) {
+                        _Os << _STD put_time(&_Time, _STATICALLY_WIDEN(_CharT, "%I:%M:%S %p"));
+                        return true;
+                    }
+                }
+                return false;
             case 'j':
                 if constexpr (_Is_specialization_v<_Ty, duration>) {
                     _Os << _STD abs(_CHRONO duration_cast<days>(_Val).count());

tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp

@@ -595,10 +595,10 @@ void test_hh_mm_ss_formatter() {
     empty_braces_helper(hh_mm_ss{65745s}, STR("18:15:45"));
 
     assert(format(STR("{:%H %I %M %S %r %R %T %p}"), hh_mm_ss{13h + 14min + 15351ms})
-           == STR("13 01 14 15.351 13:14:15 13:14 13:14:15.351 PM"));
+           == STR("13 01 14 15.351 01:14:15 PM 13:14 13:14:15.351 PM"));
 
     assert(format(STR("{:%H %I %M %S %r %R %T %p}"), hh_mm_ss{-13h - 14min - 15351ms})
-           == STR("-13 01 14 15.351 13:14:15 13:14 13:14:15.351 PM"));
+           == STR("-13 01 14 15.351 01:14:15 PM 13:14 13:14:15.351 PM"));
 
     throw_helper(STR("{}"), hh_mm_ss{24h});
     throw_helper(STR("{}"), hh_mm_ss{-24h});