Prevent users from defining instances for abstract classes.

Authored by ezyang on Mar 1 2017, 2:11 AM.

Description

Prevent users from defining instances for abstract classes.

Summary:
Fixes Trac #13068.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3254