encoding/json: difference in behavior if embed a concrete type or interface for the same value

[gopherbot]

by bsr203:

What does 'go version' print?
go version go1.2.1 darwin/amd64

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

Please see this thread https://groups.google.com/forum/#!topic/golang-nuts/oi61EuR-a3g

In this example,  http://play.golang.org/p/YdFR9Aoj5O , the struct type is embedded, and
the json marshaled output is {"id":"_id_"}

In this example, http://play.golang.org/p/wchW_HBGKD ,  the interface is embedded, and
the json marshaled output is {"Ider":{"id":"_id_"}}


What should have happened instead?
I like to get the first output ({"id":"_id_"}) with the interface
embedded. 

Please provide any additional information below.
As per rog,

Unfortunately, unless the fields are statically determinable, 
I don't think you can. 

This is arguably a bug, as the documentation says: 
"Anonymous struct fields are usually marshaled as if their inner exported 
fields were fields in the outer struct" with no caveat to say 
that only applies if the inner field is not an interface type.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.4.

[rsc]

Comment 3:

Labels changed: added documentation.

Status changed to Accepted.

[gopherbot]

Comment 4:

CL https://golang.org/cl/149570043 mentions this issue.

[rsc]

Comment 5:

This issue was closed by revision 8fd56bf.

Status changed to Fixed.