QDTP: consolidate defaultValue computation and clip to range

Previously setting a range on the QDTP that excluded 1900 would have
lead to using a default value outside the QDTP's valid range. This did
not arise in practice (QDTE supplies its own defaultValue, which is in
range, and QDT/QLocale's use of QDTP don't set a range). However, I
now want to give callers some control over the century used in
two-digit years, which shall mean they can select the defaultValue's
year; and we'll still need to ensure this is in range.

Task-number: QTBUG-46843
Change-Id: I3884853dc1d2775e1049c25f208d90b8a363d0a7
Reviewed-by: Isak Fyksen <isak.fyksen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/time/qdatetimeparser.cpp

@@ -2224,11 +2224,25 @@ QString QDateTimeParser::stateName(State s) const
     }
 }
 
+
+/*!
+    \internal
+    Compute a defaultValue to pass to parse().
+*/
+QDateTime QDateTimeParser::baseDate(const QTimeZone &zone) const
+{
+    QDateTime when = QDate(1900, 1, 1).startOfDay(zone);
+    if (const QDateTime start = getMinimum(); when < start)
+        return start;
+    if (const QDateTime end = getMaximum(); when > end)
+        return end;
+    return when;
+}
+
 // Only called when we want only one of date or time; use UTC to avoid bogus DST issues.
 bool QDateTimeParser::fromString(const QString &t, QDate *date, QTime *time) const
 {
-    QDateTime val(QDate(1900, 1, 1).startOfDay(QTimeZone::UTC));
-    const StateNode tmp = parse(t, -1, val, false);
+    const StateNode tmp = parse(t, -1, baseDate(QTimeZone::UTC), false);
     if (tmp.state != Acceptable || tmp.conflicts)
         return false;
 
@@ -2253,8 +2267,7 @@ bool QDateTimeParser::fromString(const QString &t, QDate *date, QTime *time) con
 // Only called when we want both date and time; default to local time.
 bool QDateTimeParser::fromString(const QString &t, QDateTime *datetime) const
 {
-    static const QDateTime defaultLocalTime = QDate(1900, 1, 1).startOfDay();
-    const StateNode tmp = parse(t, -1, defaultLocalTime, false);
+    const StateNode tmp = parse(t, -1, baseDate(QTimeZone::LocalTime), false);
     if (datetime)
         *datetime = tmp.value;
     return tmp.state >= Intermediate && !tmp.conflicts && tmp.value.isValid();

src/corelib/time/qdatetimeparser_p.h

@@ -203,6 +203,7 @@ private:
     };
 
     QString getAmPmText(AmPm ap, Case cs) const;
+    QDateTime baseDate(const QTimeZone &zone) const;
 
     friend class QDTPUnitTestParser;