Adding support for squash: interface. (#17)

* Adding support for squash: interface.

* fix lint issues.

mapstructure.go

@@ -1375,10 +1375,15 @@ func (d *Decoder) decodeStructFromMap(name string, dataVal, val reflect.Value) e
 			}
 
 			if squash {
-				if fieldVal.Kind() != reflect.Struct {
-					errs = append(errs, fmt.Errorf("%s: unsupported type for squash: %s", fieldType.Name, fieldVal.Kind()))
-				} else {
+				switch fieldVal.Kind() {
+				case reflect.Struct:
 					structs = append(structs, fieldVal)
+				case reflect.Interface:
+					if !fieldVal.IsNil() {
+						structs = append(structs, fieldVal.Elem().Elem())
+					}
+				default:
+					errs = append(errs, fmt.Errorf("%s: unsupported type for squash: %s", fieldType.Name, fieldVal.Kind()))
 				}
 				continue
 			}

mapstructure_test.go

@@ -113,6 +113,60 @@ type SquashOnNonStructType struct {
 	InvalidSquashType int `mapstructure:",squash"`
 }
 
+type TestInterface interface {
+	GetVfoo() string
+	GetVbarfoo() string
+	GetVfoobar() string
+}
+
+type TestInterfaceImpl struct {
+	Vfoo string
+}
+
+func (t *TestInterfaceImpl) GetVfoo() string {
+	return t.Vfoo
+}
+
+func (t *TestInterfaceImpl) GetVbarfoo() string {
+	return ""
+}
+
+func (t *TestInterfaceImpl) GetVfoobar() string {
+	return ""
+}
+
+type TestNestedInterfaceImpl struct {
+	SquashOnNestedInterfaceType `mapstructure:",squash"`
+	Vfoo                        string
+}
+
+func (t *TestNestedInterfaceImpl) GetVfoo() string {
+	return t.Vfoo
+}
+
+func (t *TestNestedInterfaceImpl) GetVbarfoo() string {
+	return t.Vbarfoo
+}
+
+func (t *TestNestedInterfaceImpl) GetVfoobar() string {
+	return t.NestedSquash.Vfoobar
+}
+
+type SquashOnInterfaceType struct {
+	TestInterface `mapstructure:",squash"`
+	Vbar          string
+}
+
+type NestedSquash struct {
+	SquashOnInterfaceType `mapstructure:",squash"`
+	Vfoobar               string
+}
+
+type SquashOnNestedInterfaceType struct {
+	NestedSquash NestedSquash `mapstructure:",squash"`
+	Vbarfoo      string
+}
+
 type Map struct {
 	Vfoo   string
 	Vother map[string]string
@@ -1051,6 +1105,147 @@ func TestDecode_SquashOnNonStructType(t *testing.T) {
 	}
 }
 
+func TestDecode_SquashOnInterfaceType(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"VFoo": "42",
+		"VBar": "43",
+	}
+
+	result := SquashOnInterfaceType{
+		TestInterface: &TestInterfaceImpl{},
+	}
+	err := Decode(input, &result)
+	if err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	res := result.GetVfoo()
+	if res != "42" {
+		t.Errorf("unexpected value for VFoo: %s", res)
+	}
+
+	res = result.Vbar
+	if res != "43" {
+		t.Errorf("unexpected value for Vbar: %s", res)
+	}
+}
+
+func TestDecode_SquashOnOuterNestedInterfaceType(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"VFoo":    "42",
+		"VBar":    "43",
+		"Vfoobar": "44",
+		"Vbarfoo": "45",
+	}
+
+	result := SquashOnNestedInterfaceType{
+		NestedSquash: NestedSquash{
+			SquashOnInterfaceType: SquashOnInterfaceType{
+				TestInterface: &TestInterfaceImpl{},
+			},
+		},
+	}
+
+	err := Decode(input, &result)
+	if err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	res := result.NestedSquash.GetVfoo()
+	if res != "42" {
+		t.Errorf("unexpected value for VFoo: %s", res)
+	}
+
+	res = result.NestedSquash.Vbar
+	if res != "43" {
+		t.Errorf("unexpected value for Vbar: %s", res)
+	}
+
+	res = result.NestedSquash.Vfoobar
+	if res != "44" {
+		t.Errorf("unexpected value for Vfoobar: %s", res)
+	}
+
+	res = result.Vbarfoo
+	if res != "45" {
+		t.Errorf("unexpected value for Vbarfoo: %s", res)
+	}
+}
+
+func TestDecode_SquashOnInnerNestedInterfaceType(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"VFoo":    "42",
+		"VBar":    "43",
+		"Vfoobar": "44",
+		"Vbarfoo": "45",
+	}
+
+	result := SquashOnInterfaceType{
+		TestInterface: &TestNestedInterfaceImpl{
+			SquashOnNestedInterfaceType: SquashOnNestedInterfaceType{
+				NestedSquash: NestedSquash{
+					SquashOnInterfaceType: SquashOnInterfaceType{
+						TestInterface: &TestInterfaceImpl{},
+					},
+				},
+			},
+		},
+	}
+
+	err := Decode(input, &result)
+	if err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	res := result.GetVfoo()
+	if res != "42" {
+		t.Errorf("unexpected value for VFoo: %s", res)
+	}
+
+	res = result.Vbar
+	if res != "43" {
+		t.Errorf("unexpected value for Vbar: %s", res)
+	}
+
+	res = result.GetVfoobar()
+	if res != "44" {
+		t.Errorf("unexpected value for Vfoobar: %s", res)
+	}
+
+	res = result.GetVbarfoo()
+	if res != "45" {
+		t.Errorf("unexpected value for Vbarfoo: %s", res)
+	}
+}
+
+func TestDecode_SquashOnNilInterfaceType(t *testing.T) {
+	t.Parallel()
+
+	input := map[string]interface{}{
+		"VFoo": "42",
+		"VBar": "43",
+	}
+
+	result := SquashOnInterfaceType{
+		TestInterface: nil,
+	}
+	err := Decode(input, &result)
+	if err != nil {
+		t.Fatalf("got an err: %s", err)
+	}
+
+	res := result.Vbar
+	if res != "43" {
+		t.Errorf("unexpected value for Vbar: %s", res)
+	}
+}
+
 func TestDecode_DecodeHook(t *testing.T) {
 	t.Parallel()