encoding/json: cannot call UnmarshalJSON on value receiver through interface #27722
Description
Noticed this inconsistency when poking around the code:
type CustomUnmarshaler struct{ p *bytes.Buffer }
func (cu CustomUnmarshaler) UnmarshalJSON(b []byte) error {
cu.p.Write(b)
return nil
}
func (cu CustomUnmarshaler) String() string {
return fmt.Sprintf("custom %v", cu.p.String())
}
func main() {
s := &struct {
F1 CustomUnmarshaler
F2 interface{}
F3 interface{}
}{
CustomUnmarshaler{new(bytes.Buffer)},
CustomUnmarshaler{new(bytes.Buffer)},
&CustomUnmarshaler{new(bytes.Buffer)},
}
json.Unmarshal([]byte(`{"F1": "F1", "F2": "F2", "F3": "F3"}`), s)
fmt.Println(s.F1)
fmt.Println(s.F2)
fmt.Println(s.F3)
}This current prints:
custom "F1"
F2
custom "F3"
I expect it to print:
custom "F1"
custom "F2"
custom "F3"
In this case, UnmarshalJSON is a method on the value receiver. This is a rare situation for methods that mutate the receiver, but is valid. I expect the method to still be called. That is, the json package should not assume that the receiver must be a pointer.
This is a very obscure use-case and I personally don't have a horse in the game to address this.