Ryan Scott authored Aug 14, 2018 and Krzysztof Gogolewski committed Aug 14, 2018

Summary:
When `\case` expressions are parsed normally, their
alternatives are marked as `CaseAlt` (which means that they are
pretty-printed without a `\` character in front of them, unlike for
lambda expressions). However, `\case` expressions created by way of
Template Haskell (in `Convert`) inconsistently designated the case
alternatives as `LambdaExpr`, causing them to be pretty-printed
poorly (as shown in #15518). The fix is simple: use `CaseAlt`
consistently.

Test Plan: make test TEST=T15518

Reviewers: goldfire, bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #15518

Differential Revision: https://phabricator.haskell.org/D5069