time: fix AddDate with nil location

AddDate now retrieves location from t.Location() to ensure that
it never calls Date with a nil location.

Added test for this bug on all Time's methods

Fixes #15852

Change-Id: Id2a222af56993f741ad0b802a2c3b89e8e463926
Reviewed-on: https://go-review.googlesource.com/23561Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>

src/time/time.go

@@ -657,7 +657,7 @@ func Until(t Time) Duration {
 func (t Time) AddDate(years int, months int, days int) Time {
 	year, month, day := t.Date()
 	hour, min, sec := t.Clock()
-	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec), t.loc)
+	return Date(year+years, month+Month(months), day+days, hour, min, sec, int(t.nsec), t.Location())
 }
 
 const (

src/time/time_test.go

@@ -1039,6 +1039,98 @@ func TestDurationHours(t *testing.T) {
 	}
 }
 
+var defaultLocTests = []struct {
+	name string
+	f    func(t1, t2 Time) bool
+}{
+	{"After", func(t1, t2 Time) bool { return t1.After(t2) == t2.After(t1) }},
+	{"Before", func(t1, t2 Time) bool { return t1.Before(t2) == t2.Before(t1) }},
+	{"Equal", func(t1, t2 Time) bool { return t1.Equal(t2) == t2.Equal(t1) }},
+
+	{"IsZero", func(t1, t2 Time) bool { return t1.IsZero() == t2.IsZero() }},
+	{"Date", func(t1, t2 Time) bool {
+		a1, b1, c1 := t1.Date()
+		a2, b2, c2 := t2.Date()
+		return a1 == a2 && b1 == b2 && c1 == c2
+	}},
+	{"Year", func(t1, t2 Time) bool { return t1.Year() == t2.Year() }},
+	{"Month", func(t1, t2 Time) bool { return t1.Month() == t2.Month() }},
+	{"Day", func(t1, t2 Time) bool { return t1.Day() == t2.Day() }},
+	{"Weekday", func(t1, t2 Time) bool { return t1.Weekday() == t2.Weekday() }},
+	{"ISOWeek", func(t1, t2 Time) bool {
+		a1, b1 := t1.ISOWeek()
+		a2, b2 := t2.ISOWeek()
+		return a1 == a2 && b1 == b2
+	}},
+	{"Clock", func(t1, t2 Time) bool {
+		a1, b1, c1 := t1.Clock()
+		a2, b2, c2 := t2.Clock()
+		return a1 == a2 && b1 == b2 && c1 == c2
+	}},
+	{"Hour", func(t1, t2 Time) bool { return t1.Hour() == t2.Hour() }},
+	{"Minute", func(t1, t2 Time) bool { return t1.Minute() == t2.Minute() }},
+	{"Second", func(t1, t2 Time) bool { return t1.Second() == t2.Second() }},
+	{"Nanosecond", func(t1, t2 Time) bool { return t1.Hour() == t2.Hour() }},
+	{"YearDay", func(t1, t2 Time) bool { return t1.YearDay() == t2.YearDay() }},
+
+	// Using Equal since Add don't modify loc using "==" will cause a fail
+	{"Add", func(t1, t2 Time) bool { return t1.Add(Hour).Equal(t2.Add(Hour)) }},
+	{"Sub", func(t1, t2 Time) bool { return t1.Sub(t2) == t2.Sub(t1) }},
+
+	//Original caus for this test case bug 15852
+	{"AddDate", func(t1, t2 Time) bool { return t1.AddDate(1991, 9, 3) == t2.AddDate(1991, 9, 3) }},
+
+	{"UTC", func(t1, t2 Time) bool { return t1.UTC() == t2.UTC() }},
+	{"Local", func(t1, t2 Time) bool { return t1.Local() == t2.Local() }},
+	{"In", func(t1, t2 Time) bool { return t1.In(UTC) == t2.In(UTC) }},
+
+	{"Local", func(t1, t2 Time) bool { return t1.Local() == t2.Local() }},
+	{"Zone", func(t1, t2 Time) bool {
+		a1, b1 := t1.Zone()
+		a2, b2 := t2.Zone()
+		return a1 == a2 && b1 == b2
+	}},
+
+	{"Unix", func(t1, t2 Time) bool { return t1.Unix() == t2.Unix() }},
+	{"UnixNano", func(t1, t2 Time) bool { return t1.UnixNano() == t2.UnixNano() }},
+
+	{"MarshalBinary", func(t1, t2 Time) bool {
+		a1, b1 := t1.MarshalBinary()
+		a2, b2 := t2.MarshalBinary()
+		return bytes.Equal(a1, a2) && b1 == b2
+	}},
+	{"GobEncode", func(t1, t2 Time) bool {
+		a1, b1 := t1.GobEncode()
+		a2, b2 := t2.GobEncode()
+		return bytes.Equal(a1, a2) && b1 == b2
+	}},
+	{"MarshalJSON", func(t1, t2 Time) bool {
+		a1, b1 := t1.MarshalJSON()
+		a2, b2 := t2.MarshalJSON()
+		return bytes.Equal(a1, a2) && b1 == b2
+	}},
+	{"MarshalText", func(t1, t2 Time) bool {
+		a1, b1 := t1.MarshalText()
+		a2, b2 := t2.MarshalText()
+		return bytes.Equal(a1, a2) && b1 == b2
+	}},
+
+	{"Truncate", func(t1, t2 Time) bool { return t1.Truncate(Hour).Equal(t2.Truncate(Hour)) }},
+	{"Round", func(t1, t2 Time) bool { return t1.Round(Hour).Equal(t2.Round(Hour)) }},
+}
+
+func TestDefaultLoc(t *testing.T) {
+	//This test verifyes that all Time's methods behaves identical if loc is set
+	//as nil or UTC
+	for _, tt := range defaultLocTests {
+		t1 := Time{}
+		t2 := Time{}.UTC()
+		if !tt.f(t1, t2) {
+			t.Errorf("Default fail on fuction: %s", tt.name)
+		}
+	}
+}
+
 func BenchmarkNow(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		t = Now()