panic: json: cannot unmarshal object into Go struct field Payload.Data of type main.Key

[aexvir]

we're still evaluating go-json as replacement for jsoniter on our setup, and during our testing I've found another issue where go-json wouldn't be able to parse valid json that both encoding/json and jsoniter parse just fine

this time it seems to be caused by using custom types with custom unmarshal functions as map keys
when attaching the debugger with a breakpoint on the UnmarshalJSON function I can't see the function being called at all, so it seems to be panicking somewhere earlier

reproducible by:

package main

import (
	"bytes"
	"fmt"

	"github.com/goccy/go-json"
)

const (
	First  = "first item"
	Second = "second item"
)

type Key string

func (key *Key) UnmarshalJSON(b []byte) error {
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}

	switch s {
	case "first":
		*key = First
	case "second":
		*key = Second
	default:
		return fmt.Errorf("option not recognized: %s", s)
	}

	return nil
}

type Value string

func (val *Value) UnmarshalJSON(b []byte) error {
	var s string
	if err := json.Unmarshal(b, &s); err != nil {
		return err
	}

	*val = Value("letter " + s)
	return nil
}

type Payload struct {
	Data map[Key]Value `json:"data"`
}

const jsondata = `
{
	"data": {
		"first": "a",
		"second": "d"
	}
}
`

func main() {
	var pld Payload

	reader := bytes.NewReader([]byte(jsondata))
	err := json.NewDecoder(reader).Decode(&pld)
	if err != nil {
		panic(err)
	}

	data, err := json.Marshal(pld)

	fmt.Println(string(data))
}

---

note: just by removing (or commenting out) the func (key *Key) UnmarshalJSON(b []byte) function will make the parsing successful, so it doesn't look like a specific issue just because of custom types as map keys

[orisano]

There is an incompatible behavior.
but encoding/json does not support UnmarshalJSON on map key type.
https://go.dev/play/p/gtPW1rnJBuG

[orisano]

encoding/json directly converts the string to KeyType. it does not call UnmarshalJSON.
https://github.com/golang/go/blob/6e63be7b69aab25ac66029e7dfec47303d3b7505/src/encoding/json/decode.go#L792

[aexvir]

oh, yep, you're right! good catch
this seems to be something from jsoniter, and I incorrectly assumed encoding/json was behaving the same

[aexvir]

but in that case I don't think that go-json should return and error; it should just ignore it as encoding/json, no?
is go-json trying to do something with the unmarshal function? or why is it erroring?

and... will this feature be supported? 😅 should I file another issue as feature request?
I think it's a nice feature, and while we can work around it by validating the strings after the unmarshaling, it would be much nicer to have it done on a single step

[aexvir]

someone smarter than me pointed out today that encoding/json actually does allow for custom unmarshaling on map keys, as long as the custom type implements the encoding.TextMarshaler interface 😅
it's documented on the official docs

I verified that the UnmarshalText function indeed gets called when using encoding/json: https://goplay.space/#FA41T64uoGy

so adding support for custom unmarshalers on map keys is needed for this library to be compliant with the std lib; allowing to use it like jsoniter would be just additional sugarcoat

edit: I just realized after providing the goplay.space link that actually that works just fine in go-json 😅
so the only issue here is that it shouldn't return an error when UnmarshalJSON is present, or just add support for that