What version of dep are you using (dep version)?

$ brew info dep
dep: stable 0.3.1 (bottled), HEAD
Go dependency management tool
https://github.com/golang/dep
/usr/local/Cellar/dep/0.3.1 (7 files, 10.8MB) *
  Poured from bottle on 2017-10-13 at 12:58:59
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/dep.rb
==> Dependencies
Required: go ✔

$ dep version
dep:
 version     : devel
 build date  :
 git hash    :
 go version  : go1.9
 go compiler : gc
 platform    : darwin/amd64

What dep command did you run?

After a dep ensure, I moved vendor/github.com/gopherjs/gopherjs outside of my project directory (gopherjs doesn't support being vendored), then I tried to regenerate it in the vendor directory using

dep ensure -update github.com/gopherjs/gopherjs

What did you expect to see?

The reappearance of vendor/github.com/gopherjs/gopherjs in my project directory.

What did you see instead?

Nothing new appeared. Running in verbose mode suggests that dep determines that nothing should be done. However, running a full dep ensure does regenerate vendor/github.com/gopherjs/gopherjs.