This issue seems to have been fixed by
193664d4 (Re-engineer caseRules
to add tagToEnum/dataToTag) but I don't think anyone realized
that.

Reviewers: simonpj, austin, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie

GHC Trac Issues: #14140

Differential Revision: https://phabricator.haskell.org/D3934