Vladislav Zavialov authored Nov 21, 2017 and Ben Gamari committed Nov 21, 2017

Summary:
The refactoring in 3f5673f3 also fixed a
previously unreported issue in the typechecker that prevented defining a
lens to a record field with a constraint. This patch adds a regression
test.

Test Plan: make test TEST=14488

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: int-e, rwbarton, thomie

GHC Trac Issues: #14488

Differential Revision: https://phabricator.haskell.org/D4213