`<chrono>` LWG-3536: leave duration unchanged on failed parse (#2037)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/chrono

@@ -4096,7 +4096,6 @@ namespace chrono {
         _NODISCARD bool _Make_duration(duration<_Rep, _Period>& _Duration_result) {
             const bool _Consistent = _Calculate_hour24();
             if (_Consistent) {
-                _Duration_result = duration<_Rep, _Period>::zero(); // TRANSITION: LWG-3536 & GH-1740
                 return _Apply_duration_fields<_Parse_tp_or_duration::_Duration>(_Duration_result);
             }
 

tests/std/tests/P0355R7_calendars_and_time_zones_io/test.cpp

@@ -221,6 +221,29 @@ void test_limits(const char* flag, const IntType min, const IntType max) {
     assert(value == TimeType{max});
 }
 
+void test_lwg_3536() {
+    // LWG-3536, "Should chrono::from_stream() assign zero to duration for failure?"
+    minutes mm{20};
+
+    {
+        istringstream iss{"2:2:30"};
+        iss >> parse(string{"%H:%M:%S"}, mm);
+        assert(iss.fail() && mm == 20min);
+    }
+
+    {
+        istringstream iss{"June"};
+        iss >> parse(string{"%B"}, mm);
+        assert(iss.fail() && mm == 20min);
+    }
+
+    {
+        istringstream iss{""};
+        iss >> parse(string{"%B"}, mm);
+        assert(iss.fail() && mm == 20min);
+    }
+}
+
 void parse_seconds() {
     seconds time;
     test_parse("1", "%S", time);
@@ -1182,6 +1205,7 @@ void parse_wchar() {
 }
 
 void test_parse() {
+    test_lwg_3536();
     parse_seconds();
     parse_minutes();
     parse_hours();