encoding/json: Unexpected marshal behavior

[alexandrevicenzi]

I created a custom MarshalJSON for my struct and it outputs the wrong struct if I have customs MarshalJSON nested in my struct.

I'm trying to understand why it doesn't work as expected. I know that there are some limitations with type redefinition but it works well if I don't nest any struct with a custom MarshalJSON.

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

1.8

Does this issue reproduce with the latest release?

Yes

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

Linux

What did you do?

https://play.golang.org/p/Mp1ot20ILA

This example is just to be able to reproduce the problem. I know it doesn't make any sense the custom Copy type, but in a real problem, I would extend Copy type by adding custom fields.

What did you expect to see?

{"value":"A"}
{"value":"B","A":{"value":"A"}}
{"value":"C","A":{"value":"A"},"B":{"value":"B","A":{"value":"A"}}}

or at least

{"value":"A"}
{"value":"B"}
{"value":"C"}

What did you see instead?

{"value":"A"}
{"value":"A"}
{"value":"C","A":{"value":"A"},"B":{"value":"A"}}

The first line is ok.
The second line is completely wrong.
The third line is almost ok.

C has two nested objects with a custom marshal and it works almost good.
B has just one and it prints the output of A.

[ianlancetaylor]

Note that we don't use the issue tracker to ask questions. Please see https://golang.org/wiki/Questions .

[alexandrevicenzi]

In my opinion, it's a bug since it works for some cases.

[dsnet]

I should note that Go 1.0 had the user's expected behavior:

{"value":"A"}
{"value":"B"}
{"value":"C"}

The current output has been the case since Go1.1.

[go101]

Maybe Go 1.0 is buggy. I feel the results of the later versions are right.

func (this *B) MarshalJSON() ([]byte, error) {
	type Copy B
	c := (*Copy)(this)
	return json.Marshal(c)
}

The type *Copy defined in (*B).MarshalJSON has one method,
this method comes from (*A).MarshalJSON.
(In a type definition, the new defined type will not retain the methods directly defined on the source type).
json.Marshal(c) will call c.A.MarshalJSON() consequently,
this is why the json.Marshal(c) in (*B).MarshalJSON will return []byte("{\"value\":\"A\"}").

The *Copy type in (*C).MarshalJSON has no methods for the collision of (*A).MarshalJSON and (*B).MarshalJSON.

[ianlancetaylor]

Doesn't seem to be anything to do here.