Add support for "whitsun +1 day", should be "easter +50 days"

This is just in case, I haven't actually seen this in the wild.

autotests/parsertest.cpp

@@ -44,6 +44,7 @@ private Q_SLOTS:
         T("easter +1 day off");
         T("easter -2 days off");
         T2("whitsun off", "easter +49 days off");
+        T2("whitsun +1 day off", "easter +50 days off");
         T("2020");
         T("2020-2021");
         T("1970-2022/2");

src/lib/openinghoursparser.y

@@ -668,7 +668,7 @@ MonthdayRangeAdditional:
 | DateFrom[D] DateOffset[O] {
     $$ = new MonthdayRange;
     $$->begin = $D;
-    $$->begin.offset = $O;
+    $$->begin.offset += $O;
     $$->end = $$->begin;
   }
 | DateFrom[F] RangeSeparator DateTo[T] {
@@ -681,7 +681,7 @@ MonthdayRangeAdditional:
 | DateFrom[F] DateOffset[OF] RangeSeparator DateTo[T] {
     $$ = new MonthdayRange;
     $$->begin = $F;
-    $$->begin.offset = $OF;
+    $$->begin.offset += $OF;
     $$->end = $T;
     if ($$->end.year == 0) { $$->end.year = $$->begin.year; }
     if ($$->end.month == 0) { $$->end.month = $$->begin.month; }
@@ -692,7 +692,7 @@ MonthdayRangeAdditional:
     $$->end = $T;
     if ($$->end.year == 0) { $$->end.year = $$->begin.year; }
     if ($$->end.month == 0) { $$->end.month = $$->begin.month; }
-    $$->end.offset = $OT;
+    $$->end.offset += $OT;
   }
 | DateFrom[F] RangeSeparator T_MONTH[M] AltMonthdayOffset[O] {
     $$ = new MonthdayRange;
@@ -707,11 +707,11 @@ MonthdayRangeAdditional:
 | DateFrom[F] DateOffset[OF] RangeSeparator DateTo[T] DateOffset[OT] {
     $$ = new MonthdayRange;
     $$->begin = $F;
-    $$->begin.offset = $OF;
+    $$->begin.offset += $OF;
     $$->end = $T;
     if ($$->end.year == 0) { $$->end.year = $$->begin.year; }
     if ($$->end.month == 0) { $$->end.month = $$->begin.month; }
-    $$->end.offset = $OT;
+    $$->end.offset += $OT;
   }
 ;
 

src/lib/selectors.cpp

@@ -296,6 +296,18 @@ bool DateOffset::operator==(DateOffset other) const
     return weekday == other.weekday && nthWeekday == other.nthWeekday && dayOffset == other.dayOffset;
 }
 
+DateOffset &DateOffset::operator+=(DateOffset other)
+{
+    // Only dayOffset really supports += (this is for whitsun)
+    dayOffset += other.dayOffset;
+    // The others can't possibly be set already
+    Q_ASSERT(weekday == 0);
+    Q_ASSERT(nthWeekday == 0);
+    weekday = other.weekday;
+    nthWeekday = other.nthWeekday;
+    return *this;
+}
+
 bool Date::operator==(Date other) const
 {
     if (variableDate != other.variableDate)

src/lib/selectors_p.h

@@ -176,6 +176,7 @@ class DateOffset
 {
 public:
     bool operator==(DateOffset other) const;
+    DateOffset &operator+=(DateOffset other);
 
     int16_t dayOffset;
     int8_t weekday;