Turn on MonadFail desugaring by default

Summary:
This contains two commits:

----

Make GHC's code-base compatible w/ `MonadFail`

There were a couple of use-sites which implicitly used pattern-matches
in `do`-notation even though the underlying `Monad` didn't explicitly
support `fail`

This refactoring turns those use-sites into explicit case
discrimations and adds an `MonadFail` instance for `UniqSM`
(`UniqSM` was the worst offender so this has been postponed for a
follow-up refactoring)

---

Turn on MonadFail desugaring by default

This finally implements the phase scheduled for GHC 8.6 according to

https://prime.haskell.org/wiki/Libraries/Proposals/MonadFail#Transitionalstrategy

This also preserves some tests that assumed MonadFail desugaring to be
active; all ghc boot libs were already made compatible with this
`MonadFail` long ago, so no changes were needed there.

Test Plan: Locally performed ./validate --fast

Reviewers: bgamari, simonmar, jrtc27, RyanGlScott

Reviewed By: bgamari

Subscribers: bgamari, RyanGlScott, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D5028

1 files changed, 1 insertions, 1 deletions

diff --git a/testsuite/tests/rebindable/rebindable1.hs b/testsuite/tests/rebindable/rebindable1.hs
index 8fcc5d2697..fcbe52fbc1 100644
--- a/testsuite/tests/rebindable/rebindable1.hs
+++ b/testsuite/tests/rebindable/rebindable1.hs
@@ -1,5 +1,5 @@
 {-# OPTIONS_GHC -Wno-missing-monadfail-instances #-}
-{-# LANGUAGE RebindableSyntax, NPlusKPatterns #-}
+{-# LANGUAGE RebindableSyntax, NPlusKPatterns, NoMonadFailDesugaring #-}
 
 module RebindableCase1 where
         {