Test case for #167

issue167_test.go

@@ -0,0 +1,64 @@
+package mergo_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/imdario/mergo"
+)
+
+// This test has been stripped to minimally reproduce the core issue of #167. Consider the playground link for an in-depth analysis.
+
+type Issue167A struct{}
+
+type Issue167InMap struct {
+	As []*Issue167A
+}
+
+type Issue167ValueMap struct {
+	InMap map[string]Issue167InMap
+}
+
+type Issue167Transformer struct{}
+
+func (t Issue167Transformer) Transformer(typ reflect.Type) func(dst, src reflect.Value) error {
+	switch typ {
+	case reflect.TypeOf([]*Issue167A{}):
+		return t.transform
+	}
+
+	return nil
+}
+
+func (t Issue167Transformer) transform(dst, src reflect.Value) error {
+	if !dst.CanSet() {
+		return fmt.Errorf("dst.CanSet() is false, so we're unable to Set() the new value later")
+	}
+
+	dstReal := dst.Interface().([]*Issue167A)
+	dst.Set(reflect.ValueOf(dstReal))
+
+	return nil
+}
+
+var Issue167TestData = []struct {
+	Dst Issue167ValueMap
+	Src Issue167ValueMap
+}{
+	{
+		// Each As must have at least an element
+		Dst: Issue167ValueMap{InMap: map[string]Issue167InMap{"foo": {As: []*Issue167A{{}}}}},
+		Src: Issue167ValueMap{InMap: map[string]Issue167InMap{"foo": {As: []*Issue167A{{}}}}},
+	},
+}
+
+func TestIssue167MapWithValue(t *testing.T) {
+	// And additionally should not panic with every test case
+	for _, data := range Issue167TestData {
+		err := mergo.Merge(&data.Dst, data.Src, mergo.WithTransformers(Issue167Transformer{}))
+		if err != nil {
+			t.Errorf("Error while merging %s", err)
+		}
+	}
+}