github.com/mailgun/mailgun-go: go fmt will not reformat sources with build tags

[gopherbot]

by sam.falvo:

What does 'go version' print?
go version go1.2 darwin/amd64

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

0.  export GOPATH=$(pwd)
1.  go get github.com/mailgun/mailgun-go
2.  (cd src/github.com/mailgun/mailgun-go && git checkout
create-acceptance-tests)
3.  go fmt github.com/mailgun/mailgun-go/acceptance

What happened?
It prints:
"""
can't load package: package github.com/mailgun/mailgun-go/acceptance: no buildable Go
source files in /Users/sfalvo/go/mailgun-go/src/github.com/mailgun/mailgun-go/acceptance
"""

What should have happened instead?
It should have reformatted the sources, printing the list of filenames it affected;
otherwise, it should have just returned to the shell with no output, indicating no
changes have been made, just as it does for 'normal' sources.

[gopherbot]

Comment 1 by sam.falvo:

Tv` from IRC (#go-nuts) verified the issue with a simpler to reproduce instruction than
I posted above.  Hopefully this will be at least as helpful if not more-so:
$ mkdir z && cd z && echo -e '// +build nope\n\npackage foo' >bar.go && go fmt -n .

[gopherbot]

Comment 2:

From https://golang.org/issue/4007?c=10
(sorry for the cross-talk; now that this issue exists, let's use only this):
rsc@golang.org
> no, it is working. the go command requires that there be at least one
> non-ignored go file in a directory. echo package foo >baz.go and then run
> go fmt -n .

[adg]

Comment 3:

Status changed to Duplicate.

Merged into issue #4007.