rwbarton authored Feb 05, 2017 and Ben Gamari committed Feb 05, 2017

This fixes the easy part of #7102 by removing the logic that lets the
user replace a type family instance with a new one with the same LHS.
As discussed on that ticket, this is unsound in general. Better to have
the user redefine the type family from scratch.

The example from comment:7 involving loading modules into ghci is not
fixed yet; it actually doesn't rely on the instances having the same LHS.
This commit adds an expect_broken test for that example as well.

Test Plan: T7102a for the fix; T7102 is the test not fixed yet

Reviewers: dfeuer, austin, bgamari, goldfire

Reviewed By: dfeuer

Subscribers: dfeuer, thomie

Differential Revision: https://phabricator.haskell.org/D2994