Suggestion (non-blocking): I think it could be beneficial to make this a package-private type and expose the fields via functions.

Benefit is that a caller can't access the underlying ResponseWriter (bypassing StatusWriter), but can still test for the values via if s, ok := rw.(interface{Status() int}); ok { s.Status() }

I'm not sure I'm following. Do you just want to create a StatusWriter interface and unexport the current struct? Alternatively we could simply unexport the ResponseWriter on the struct.

Ah, sorry it was unclear, I was suggesting two changes:

1. Unexport ResponseWriter => responseWriter
2. Change fields from Status => func(*statusWriter) Status() int

Number 1 was the main thing (to protect from direct ResponseWriter access), number 2 was a little extra which would allow the values to be inspected outside the package, if the need ever arises. But not necessary by any means.

In the case of 2, my example of if s, ok := rw.(interface{Status() int}); ok { s.Status() } was hinting at that we don't need to export anything from this package. If that need ever arises (inspect value outside package), it could be done with an inline interface.

I could've omitted that part though, I don't think it will be needed. 😄

Technically this will be true once we hit Write below, a later call to WriteHeader might overwrite the actual status otherwise.