Fixed handling of merging null #1501

pkg/yqlib/doc/operators/multiply-merge.md

@@ -499,3 +499,43 @@ b: !goat
   dog: woof
 ```
 
+## Merging a null with a map
+Running
+```bash
+yq --null-input 'null * {"some": "thing"}'
+```
+will output
+```yaml
+some: thing
+```
+
+## Merging a map with null
+Running
+```bash
+yq --null-input '{"some": "thing"} * null'
+```
+will output
+```yaml
+some: thing
+```
+
+## Merging an null with an array
+Running
+```bash
+yq --null-input 'null * ["some"]'
+```
+will output
+```yaml
+- some
+```
+
+## Merging an array with null
+Running
+```bash
+yq --null-input '["some"] * null'
+```
+will output
+```yaml
+- some
+```
+

pkg/yqlib/operator_multiply.go

@@ -60,8 +60,14 @@ func multiply(preferences multiplyPreferences) func(d *dataTreeNavigator, contex
 		log.Debugf("Multiplying LHS: %v", lhs.Node.Tag)
 		log.Debugf("-          RHS: %v", rhs.Node.Tag)
 
-		if lhs.Node.Kind == yaml.MappingNode && rhs.Node.Kind == yaml.MappingNode ||
-			(lhs.Node.Kind == yaml.SequenceNode && rhs.Node.Kind == yaml.SequenceNode) {
+		if rhs.Node.Tag == "!!null" {
+			return lhs.Copy()
+		}
+
+		if (lhs.Node.Kind == yaml.MappingNode && rhs.Node.Kind == yaml.MappingNode) ||
+			(lhs.Node.Tag == "!!null" && rhs.Node.Kind == yaml.MappingNode) ||
+			(lhs.Node.Kind == yaml.SequenceNode && rhs.Node.Kind == yaml.SequenceNode) ||
+			(lhs.Node.Tag == "!!null" && rhs.Node.Kind == yaml.SequenceNode) {
 			var newBlank = CandidateNode{}
 			err := copier.CopyWithOption(&newBlank, lhs, copier.Option{IgnoreEmpty: true, DeepCopy: true})
 			if err != nil {

pkg/yqlib/operator_multiply_test.go

@@ -579,6 +579,41 @@ var multiplyOperatorScenarios = []expressionScenario{
 			"D0, P[], (doc)::a: {a: apple is included, b: cool.}\n",
 		},
 	},
+	{
+		description: "Merging a null with a map",
+		expression:  `null * {"some": "thing"}`,
+		expected: []string{
+			"D0, P[], (!!map)::some: thing\n",
+		},
+	},
+	{
+		description: "Merging a map with null",
+		expression:  `{"some": "thing"} * null`,
+		expected: []string{
+			"D0, P[], (!!map)::some: thing\n",
+		},
+	},
+	{
+		description: "Merging an null with an array",
+		expression:  `null * ["some"]`,
+		expected: []string{
+			"D0, P[], (!!seq)::- some\n",
+		},
+	},
+	{
+		description: "Merging an array with null",
+		expression:  `["some"] * null`,
+		expected: []string{
+			"D0, P[], (!!seq)::- some\n",
+		},
+	},
+	{
+		skipDoc:    true,
+		expression: `null * null`,
+		expected: []string{
+			"D0, P[], (!!null)::null\n",
+		},
+	},
 }
 
 func TestMultiplyOperatorScenarios(t *testing.T) {