Issue #149 fixed

go.mod

@@ -2,4 +2,7 @@ module github.com/aivetech/mergo
 
 go 1.13
 
-require gopkg.in/yaml.v2 v2.3.0
+require (
+	github.com/imdario/mergo v0.3.9 // indirect
+	gopkg.in/yaml.v2 v2.3.0
+)

go.sum

@@ -1,3 +1,5 @@
+github.com/imdario/mergo v0.3.9 h1:UauaLniWCFHWd+Jp9oCEkTBj8VO/9DKg3PV3VCNMDIg=
+github.com/imdario/mergo v0.3.9/go.mod h1:2EnlNZ0deacrJVfApfmtdGgDfMuh/nq6Ok1EcJh5FfA=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
 gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

issue149_test.go

@@ -0,0 +1,37 @@
+package mergo
+
+import (
+	"testing"
+)
+
+type user struct {
+	Name string
+}
+
+type token struct {
+	User  *user
+	Token *string
+}
+
+func TestIssue149(t *testing.T) {
+	dest := &token{
+		User: &user{
+			Name: "destination",
+		},
+		Token: nil,
+	}
+	tokenValue := "Issue149"
+	src := &token{
+		User:  nil,
+		Token: &tokenValue,
+	}
+	if err := Merge(dest, src, WithOverwriteWithEmptyValue); err != nil {
+		t.Error(err)
+	}
+	if dest.User != nil {
+		t.Errorf("expected nil User, got %q", dest.User)
+	}
+	if dest.Token == nil {
+		t.Errorf("expected not nil Token, got %q", *dest.Token)
+	}
+}

merge.go

@@ -102,6 +102,14 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		if dst.IsNil() && !src.IsNil() {
 			dst.Set(reflect.MakeMap(dst.Type()))
 		}
+
+		if src.Kind() != reflect.Map {
+			if overwrite {
+				dst.Set(src)
+			}
+			return
+		}
+
 		for _, key := range src.MapKeys() {
 			srcElement := src.MapIndex(key)
 			if !srcElement.IsValid() {
@@ -219,6 +227,9 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 		fallthrough
 	case reflect.Interface:
 		if src.IsNil() {
+			if overwriteWithEmptySrc && dst.CanSet() && src.Type().AssignableTo(dst.Type()) {
+				dst.Set(src)
+			}
 			break
 		}
 
@@ -249,6 +260,7 @@ func deepMerge(dst, src reflect.Value, visited map[uintptr]*visit, depth int, co
 			}
 			break
 		}
+
 		if dst.IsNil() || overwrite {
 			if dst.CanSet() && (overwrite || isEmptyValue(dst)) {
 				dst.Set(src)

mergo_test.go

@@ -529,7 +529,7 @@ func TestMergeUsingStructAndMap(t *testing.T) {
 				t.Error(err)
 			}
 			if !reflect.DeepEqual(tc.target, tc.output) {
-				t.Errorf("Test failed:\ngot  :\n%#v\n\nwant :\n%#v\n\n", tc.target, tc.output)
+				t.Errorf("Test failed:\ngot  :\n%+v\n\nwant :\n%+v\n\n", tc.target.Params, tc.output.Params)
 			}
 		})
 	}