encoding/json: custom MarshalJSON() on anonymous field blocks encoding of sibling struct fields

[BourgeoisBear]

What version of Go are you using (go version)?

$ go version
go version go1.14.4 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"

What did you do?

When I JSON encode a struct that embeds another struct (ICustom) as an anonymous field, and that struct (ICustom) implements MarshalJSON(), only ICustom is encoded, and other fields are omitted.

When I do the same thing, but where the embedded struct doesn't implement MarshalJSON(), all fields are included.

See: https://play.golang.org/p/VKH--_G432v

What did you expect to see?

The same output for both, given the code in the example.

What did you see instead?

Sibling fields omitted.

[mvdan]

This seems like a dupe of #39175.

[BourgeoisBear]

@mvdan in what way? #39715 is an unmarshaling issue. This is a marshaling one.

[mvdan]

It seems like the same root issue. Your method is being promoted to the main type, so it's being used to marshal the entire parent type, not just the embedded one. This is how methods in Go work.

[BourgeoisBear]

That explains it. Thanks.

[BourgeoisBear]

For the next person with this problem: you can embed a 2nd struct implementing a custom MarshalJSON(), and sibling fields will encode again. I made a dummy struct just for this purpose:

type JSON_Hack int

func (V JSON_Hack) MarshalJSON() ([]byte, error) {
	return []byte("null"), nil
}

type Example struct {
   Embed1WithCustomMarshaller
   JSON_Hack `json:"-"`
   Sibling1   int
   Sibling2   string
}