From 710dbb74c9c9e0f826b254418aaafd596f8824cc Mon Sep 17 00:00:00 2001
From: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
Date: Wed, 3 Jun 2026 15:31:04 +0200
Subject: [PATCH] promql/parser: fix symmetric fill detection to compare values
 not pointers

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
---
 promql/parser/printer.go      | 2 +-
 promql/parser/printer_test.go | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/promql/parser/printer.go b/promql/parser/printer.go
index e81e4ca7b2..703dd095d4 100644
--- a/promql/parser/printer.go
+++ b/promql/parser/printer.go
@@ -174,7 +174,7 @@ func (node *BinaryExpr) getMatchingStr() string {
 		}
 
 		if vm.FillValues.LHS != nil || vm.FillValues.RHS != nil {
-			if vm.FillValues.LHS == vm.FillValues.RHS {
+			if vm.FillValues.LHS != nil && vm.FillValues.RHS != nil && *vm.FillValues.LHS == *vm.FillValues.RHS {
 				matching += fmt.Sprintf(" fill (%v)", *vm.FillValues.LHS)
 			} else {
 				if vm.FillValues.LHS != nil {
diff --git a/promql/parser/printer_test.go b/promql/parser/printer_test.go
index 786109ab9d..8be35d9a48 100644
--- a/promql/parser/printer_test.go
+++ b/promql/parser/printer_test.go
@@ -130,6 +130,10 @@ func TestExprString(t *testing.T) {
 			in:  `a + fill_left(-23) fill_right(42) b`,
 			out: `a + fill_left (-23) fill_right (42) b`,
 		},
+		{
+			in:  `a + fill_left(5) fill_right(5) b`,
+			out: `a + fill (5) b`,
+		},
 		{
 			in:  `a + on(b) group_left fill(-23) c`,
 			out: `a + on (b) group_left () fill (-23) c`,