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, 6 insertions, 2 deletions

diff --git a/compiler/deSugar/Coverage.hs b/compiler/deSugar/Coverage.hs
index b5c18e5d66..99ba96755f 100644
--- a/compiler/deSugar/Coverage.hs
+++ b/compiler/deSugar/Coverage.hs
@@ -292,11 +292,15 @@ addTickLHsBind (L pos (funBind@(FunBind { fun_id = (L _ id)  }))) = do
   tickish <- tickishType `liftM` getEnv
   if inline && tickish == ProfNotes then return (L pos funBind) else do
 
-  (fvs, mg@(MG { mg_alts = matches' })) <-
+  (fvs, mg) <-
         getFreeVars $
         addPathEntry name $
         addTickMatchGroup False (fun_matches funBind)
 
+  case mg of
+    MG {} -> return ()
+    _     -> panic "addTickLHsBind"
+
   blackListed <- isBlackListed pos
   exported_names <- liftM exports getEnv
 
@@ -315,7 +319,7 @@ addTickLHsBind (L pos (funBind@(FunBind { fun_id = (L _ id)  }))) = do
                 return Nothing
 
   let mbCons = maybe Prelude.id (:)
-  return $ L pos $ funBind { fun_matches = mg { mg_alts = matches' }
+  return $ L pos $ funBind { fun_matches = mg
                            , fun_tick = tick `mbCons` fun_tick funBind }
 
    where