promql: fix stale matchedSigs maps breaking fill_left in range queries

resetMatchedSigs used clear() to recycle inner maps, but the fill-LHS
loop checked matchedSigs[sigOrd] \!= nil to decide whether an RHS sample
was already matched. A cleared but non-nil map from a previous timestep
caused a false positive, skipping the fill and producing missing samples
in range queries using group_left fill_left or group_right fill_right.

Fix by restoring clear() for map reuse and changing the check to
len(matchedSigs[sigOrd]) > 0, which correctly treats an empty map as
unmatched while preserving the allocation-reuse across timesteps.

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>

promql/engine.go

@@ -3221,7 +3221,7 @@ func (ev *evaluator) VectorBinop(op parser.ItemType, lhs, rhs Vector, matching *
 			sigOrd := rhsh[i].sigOrdinal
 
 			if (matching.Card == parser.CardOneToOne && matchedSigsPresent[sigOrd]) ||
-				(matching.Card != parser.CardOneToOne && matchedSigs[sigOrd] != nil) {
+				(matching.Card != parser.CardOneToOne && len(matchedSigs[sigOrd]) > 0) {
 				continue // Already matched.
 			}
 			ls := Sample{

promql/promqltest/testdata/fill-modifier.test

@@ -309,6 +309,21 @@ eval range from 0 to 20m step 5m intermittent_left + fill_left(0) intermittent_r
   {label="b"} 100 _ 300 _ 500
   {label="c"} 1000 _ _ 4000 5000
 
+# Regression test: group_left fill_left in a range query must apply fill values
+# at every timestep, even when the RHS sigOrd was matched at a previous timestep.
+# resetMatchedSigs previously used clear() which left non-nil empty maps, causing
+# the fill-LHS path to incorrectly skip unmatched RHS samples on subsequent timesteps.
+clear
+
+load 5m
+  matched_lhs{key="a"}  1  _  3
+  matched_rhs{key="a"}  10 20 30
+  matched_rhs{key="b"}  100 200 300
+
+eval range from 0 to 10m step 5m matched_lhs + on(key) group_left fill_left(0) matched_rhs
+  {key="a"} 11 20 33
+  {key="b"} 100 200 300
+
 # ---------- fill with vectors where one side is empty ----------
 
 clear