Correctly handle on() in alerts. (#2096)

Fixes #2082

promql/printer.go

@@ -163,7 +163,7 @@ func (node *BinaryExpr) String() string {
 
 	matching := ""
 	vm := node.VectorMatching
-	if vm != nil && len(vm.MatchingLabels) > 0 {
+	if vm != nil && (len(vm.MatchingLabels) > 0 || vm.On) {
 		if vm.On {
 			matching = fmt.Sprintf(" ON(%s)", vm.MatchingLabels)
 		} else {

promql/printer_test.go

@@ -59,6 +59,10 @@ func TestExprString(t *testing.T) {
 	inputs := []struct {
 		in, out string
 	}{
+		{
+			in:  `sum(task:errors:rate10s{job="s"}) BY ()`,
+			out: `sum(task:errors:rate10s{job="s"})`,
+		},
 		{
 			in: `sum(task:errors:rate10s{job="s"}) BY (code)`,
 		},
@@ -77,6 +81,9 @@ func TestExprString(t *testing.T) {
 		{
 			in: `count_values("value", task:errors:rate10s{job="s"})`,
 		},
+		{
+			in: `a - ON() c`,
+		},
 		{
 			in: `a - ON(b) c`,
 		},
@@ -92,6 +99,10 @@ func TestExprString(t *testing.T) {
 		{
 			in: `a - IGNORING(b) c`,
 		},
+		{
+			in:  `a - IGNORING() c`,
+			out: `a - c`,
+		},
 		{
 			in: `up > BOOL 0`,
 		},