Richard Eisenberg authored Dec 26, 2017 and Ben Gamari committed Dec 28, 2017

Previously, we were inexplicably not applying an instantiating
substitution to arguments in non-prenex types. It's amazing this
has been around for so long! I guess there aren't a lot of non-prenex
types around.

test case: typecheck/should_fail/T14618

(cherry picked from commit 722a6584)