niteria authored Mar 29, 2016 and Ben Gamari committed Mar 29, 2016

`ty` doesn't have to be a closed type, so we need to add its
free vars to the in-scope set. They don't seem to be
available anywhere nearby, so we have to compute them.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie, simonmar

Differential Revision: https://phabricator.haskell.org/D2042

GHC Trac Issues: #11371