Ryan Scott authored Oct 01, 2016 and Ben Gamari committed Oct 01, 2016

There was an awful leak where GHC permitted standalone `deriving`
declarations to create instances for unboxed sum or tuple types. This
fortifies the checks that GHC performs to catch this scenario and give
an appropriate error message.

Fixes #11509.

Test Plan: ./validate

Reviewers: goldfire, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2557

GHC Trac Issues: #11509