time: don't panic when stringifying Weekday

Fixes #24692

Change-Id: I14058cd3968d08fbcfc275f1b13b6dba9e3c5068
Reviewed-on: https://go-review.googlesource.com/106535
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/time.go

@@ -323,7 +323,14 @@ var days = [...]string{
 }
 
 // String returns the English name of the day ("Sunday", "Monday", ...).
-func (d Weekday) String() string { return days[d] }
+func (d Weekday) String() string {
+	if Sunday <= d && d <= Saturday {
+		return days[d]
+	}
+	buf := make([]byte, 20)
+	n := fmtInt(buf, uint64(d))
+	return "%!Weekday(" + string(buf[n:]) + ")"
+}
 
 // Computations on time.
 //

src/time/time_test.go

@@ -1319,6 +1319,16 @@ func TestZeroMonthString(t *testing.T) {
 	}
 }
 
+// Issue 24692: Out of range weekday panics
+func TestWeekdayString(t *testing.T) {
+	if got, want := Weekday(Tuesday).String(), "Tuesday"; got != want {
+		t.Errorf("Tuesday weekday = %q; want %q", got, want)
+	}
+	if got, want := Weekday(14).String(), "%!Weekday(14)"; got != want {
+		t.Errorf("14th weekday = %q; want %q", got, want)
+	}
+}
+
 func TestReadFileLimit(t *testing.T) {
 	const zero = "/dev/zero"
 	if _, err := os.Stat(zero); err != nil {