fix custom marshal for map key

encode_test.go

@@ -2605,3 +2605,18 @@ func TestIssue386(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+type customMapKey string
+
+func (b customMapKey) MarshalJSON() ([]byte, error) {
+	return []byte("[]"), nil
+}
+
+func TestCustomMarshalForMapKey(t *testing.T) {
+	m := map[customMapKey]string{customMapKey("skipcustom"): "test"}
+	expected, err := stdjson.Marshal(m)
+	assertErr(t, err)
+	got, err := json.Marshal(m)
+	assertErr(t, err)
+	assertEq(t, "custom map key", string(expected), string(got))
+}

internal/encoder/compiler.go

@@ -506,8 +506,6 @@ func (c *Compiler) listElemCode(typ *runtime.Type) (Code, error) {
 
 func (c *Compiler) mapKeyCode(typ *runtime.Type) (Code, error) {
 	switch {
-	case c.implementsMarshalJSON(typ):
-		return c.marshalJSONCode(typ)
 	case c.implementsMarshalText(typ):
 		return c.marshalTextCode(typ)
 	}