Nicer error message when trying to use merge anchor tags other than maps #1184

pkg/yqlib/operator_traverse_path.go

@@ -309,6 +309,9 @@ func doTraverseMap(newMatches *orderedmap.OrderedMap, candidate *CandidateNode,
 func traverseMergeAnchor(newMatches *orderedmap.OrderedMap, originalCandidate *CandidateNode, value *yaml.Node, wantedKey string, prefs traversePreferences, splat bool) error {
 	switch value.Kind {
 	case yaml.AliasNode:
+		if value.Alias.Kind != yaml.MappingNode {
+			return fmt.Errorf("can only use merge anchors with maps (!!map), but got %v", value.Alias.Tag)
+		}
 		candidateNode := originalCandidate.CreateReplacement(value.Alias)
 		return doTraverseMap(newMatches, candidateNode, wantedKey, prefs, splat)
 	case yaml.SequenceNode:

pkg/yqlib/operator_traverse_path_test.go

@@ -26,6 +26,15 @@ foobar:
   thing: foobar_thing
 `
 
+// cannot use merge anchors with arrays
+var badAliasSample = `
+_common: &common-docker-file
+  - FROM ubuntu:18.04
+
+steps:
+  <<: *common-docker-file
+`
+
 var traversePathOperatorScenarios = []expressionScenario{
 	{
 		skipDoc:    true,
@@ -529,6 +538,12 @@ var traversePathOperatorScenarios = []expressionScenario{
 			"D0, P[a 2], (!!str)::c\n",
 		},
 	},
+	{
+		skipDoc:       true,
+		document:      badAliasSample,
+		expression:    ".steps[]",
+		expectedError: "can only use merge anchors with maps (!!map), but got !!seq",
+	},
 }
 
 func TestTraversePathOperatorScenarios(t *testing.T) {