Map keys of custom types should serialize using MarshalText when available

reflect_map.go

@@ -49,6 +49,33 @@ func decoderOfMapKey(ctx *ctx, typ reflect2.Type) ValDecoder {
 return decoder
 }
 }
+
+ptrType := reflect2.PtrTo(typ)
+if ptrType.Implements(unmarshalerType) {
+return &referenceDecoder{
+&unmarshalerDecoder{
+valType: ptrType,
+},
+}
+}
+if typ.Implements(unmarshalerType) {
+return &unmarshalerDecoder{
+valType: typ,
+}
+}
+if ptrType.Implements(textUnmarshalerType) {
+return &referenceDecoder{
+&textUnmarshalerDecoder{
+valType: ptrType,
+},
+}
+}
+if typ.Implements(textUnmarshalerType) {
+return &textUnmarshalerDecoder{
+valType: typ,
+}
+}
+
 switch typ.Kind() {
 case reflect.String:
 return decoderOfType(ctx, reflect2.DefaultTypeOfKind(reflect.String))
@@ -63,31 +90,6 @@ func decoderOfMapKey(ctx *ctx, typ reflect2.Type) ValDecoder {
 typ = reflect2.DefaultTypeOfKind(typ.Kind())
 return &numericMapKeyDecoder{decoderOfType(ctx, typ)}
 default:
-ptrType := reflect2.PtrTo(typ)
-if ptrType.Implements(unmarshalerType) {
-return &referenceDecoder{
-&unmarshalerDecoder{
-valType: ptrType,
-},
-}
-}
-if typ.Implements(unmarshalerType) {
-return &unmarshalerDecoder{
-valType: typ,
-}
-}
-if ptrType.Implements(textUnmarshalerType) {
-return &referenceDecoder{
-&textUnmarshalerDecoder{
-valType: ptrType,
-},
-}
-}
-if typ.Implements(textUnmarshalerType) {
-return &textUnmarshalerDecoder{
-valType: typ,
-}
-}
 return &lazyErrorDecoder{err: fmt.Errorf("unsupported map key type: %v", typ)}
 }
 }
@@ -103,6 +105,19 @@ func encoderOfMapKey(ctx *ctx, typ reflect2.Type) ValEncoder {
 return encoder
 }
 }
+
+if typ == textMarshalerType {
+return &directTextMarshalerEncoder{
+stringEncoder: ctx.EncoderOf(reflect2.TypeOf("")),
+}
+}
+if typ.Implements(textMarshalerType) {
+return &textMarshalerEncoder{
+valType: typ,
+stringEncoder: ctx.EncoderOf(reflect2.TypeOf("")),
+}
+}
+
 switch typ.Kind() {
 case reflect.String:
 return encoderOfType(ctx, reflect2.DefaultTypeOfKind(reflect.String))
@@ -117,17 +132,6 @@ func encoderOfMapKey(ctx *ctx, typ reflect2.Type) ValEncoder {
 typ = reflect2.DefaultTypeOfKind(typ.Kind())
 return &numericMapKeyEncoder{encoderOfType(ctx, typ)}
 default:
-if typ == textMarshalerType {
-return &directTextMarshalerEncoder{
-stringEncoder: ctx.EncoderOf(reflect2.TypeOf("")),
-}
-}
-if typ.Implements(textMarshalerType) {
-return &textMarshalerEncoder{
-valType: typ,
-stringEncoder: ctx.EncoderOf(reflect2.TypeOf("")),
-}
-}
 if typ.Kind() == reflect.Interface {
 return &dynamicMapKeyEncoder{ctx, typ}
 }

value_tests/map_test.go

@@ -31,6 +31,7 @@ func init() {
 map[string]*json.RawMessage{"hello": pRawMessage(json.RawMessage("[]"))},
 map[Date]bool{{}: true},
 map[Date2]bool{{}: true},
+map[customKey]string{customKey(1): "bar"},
 )
 unmarshalCases = append(unmarshalCases, unmarshalCase{
 ptr: (*map[string]string)(nil),
@@ -55,6 +56,9 @@ func init() {
  "2018-12-13": true,
  "2018-12-14": true
 	}`,
+}, unmarshalCase{
+ptr: (*map[customKey]string)(nil),
+input: `{"foo": "bar"}`,
 })
 }
 
@@ -115,3 +119,14 @@ func (d Date2) UnmarshalJSON(b []byte) error {
 func (d Date2) MarshalJSON() ([]byte, error) {
 return []byte(d.Time.Format("2006-01-02")), nil
 }
+
+type customKey int32
+
+func (c customKey) MarshalText() ([]byte, error) {
+return []byte("foo"), nil
+}
+
+func (c *customKey) UnmarshalText(value []byte) error {
+*c = 1
+return nil
+}