Hmmm, both "cabal install" and "cabal sdist" happened in the same directory without any sandboxes, see the logs in the OP and the config at https://github.com/haskell-opengl/OpenGL/blob/master/.travis.yml. If the sdist really magically happens elsewhere, I consider this a bug in cabal. In general, cabal's behavior is a bit opaque IMHO...