fix `treat_as_floating_point_v<_DurationType>` (#2638)

Co-authored-by: Daniel Krügler <daniel.kruegler@gmail.com>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/chrono

@@ -3387,14 +3387,16 @@ namespace chrono {
             if constexpr (_Can_rep_sec) {
                 if (_Used & _F_sec) {
                     if (_Subsecond) {
-                        using _CastedType = duration<int64_t, typename _DurationType::period>;
                         const _SubsecondType _Sub{*_Subsecond};
-                        if (treat_as_floating_point_v<_DurationType> //
-                            || _CHRONO duration_cast<_CastedType>(_Sub) == _Sub) {
-                            _Result += _CHRONO duration_cast<_DurationType>(_Sub);
-                        } else {
-                            return false;
+
+                        if constexpr (!treat_as_floating_point_v<typename _DurationType::rep>) {
+                            using _CastedType = duration<int64_t, typename _DurationType::period>;
+                            if (_CHRONO duration_cast<_CastedType>(_Sub) != _Sub) {
+                                return false;
+                            }
                         }
+
+                        _Result += _CHRONO duration_cast<_DurationType>(_Sub);
                     }
 
                     if (_Second) {

tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp

@@ -259,6 +259,10 @@ void parse_seconds() {
     test_parse("0.400000000000000002", "%S", time_atto);
     assert((time_atto == duration<int64_t, deci>{4} + duration<int64_t, atto>{2}));
 
+    duration<float, ratio<1, 25>> time_float;
+    test_parse("0.33", "%S", time_float);
+    assert((time_float == duration<float, ratio<1, 25>>{8.25f}));
+
     fail_parse("1.2 1.3", "%S %S", time_ms);
     fail_parse("1.2 2.2", "%S %S", time_ms);
 }