Use case:

I have some example code (a package main with +build sometag) that is
generated. I would like to use go generate rather than have additional
instructions about which script in the directory was used to create the
example.

It would be nice to be able to go generate -tags sometag to only
generate the examples, however it seems that in the absence of a
specific file target go generate parses the entire directory without
examining tags.

See discussion at https://groups.google.com/d/topic/golang-nuts/uQy1hppWsZE/discussion