If it's a valid bug, I can fix it. I think about or checking for interface implementation beforehand, or mv Kind() to Type(). But the second variant seems quite dangerous.

cc @rsc

See the discussion when TextMarshaler support was added for map keys: #12146. There was concerns about backwards compatibility for existing kind strings that implement TextMarshaler.

This may have to be a Go2 request.

Oh, I have tried to find discussion by "TextMarshaler" keyword, but failed. Thanks!

So what shoud I do now? Add a go2 request explicitly?

This is mentioned in the documentation (https://tip.golang.org/pkg/encoding/json/#Marshal):

The map keys are sorted and used as JSON object keys by applying the following rules, subject to the UTF-8 coercion described for string values above:

• string keys are used directly
• encoding.TextMarshalers are marshaled
• integer keys are converted to strings

We could certainly be more explicit about the fact that “string keys” means “keys of any string type” rather than “keys of type string exactly”.

I think it would help. Because basically you can not put your string based type instead a string without an explicit conversion.

Change https://golang.org/cl/188417 mentions this issue: encoding/json: clarify Marshal behavior for string keys of maps