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

2 files changed, 10 insertions, 5 deletions

diff --git a/compiler/ghci/ByteCodeAsm.hs b/compiler/ghci/ByteCodeAsm.hs
index f7cea3b567..476a9b2efd 100644
--- a/compiler/ghci/ByteCodeAsm.hs
+++ b/compiler/ghci/ByteCodeAsm.hs
@@ -125,9 +125,12 @@ mallocStrings hsc_env ulbcos = do
     return bco { unlinkedBCOLits = lits, unlinkedBCOPtrs = ptrs }
 
   spliceLit (BCONPtrStr _) = do
-    (RemotePtr p : rest) <- get
-    put rest
-    return (BCONPtrWord (fromIntegral p))
+    rptrs <- get
+    case rptrs of
+      (RemotePtr p : rest) -> do
+        put rest
+        return (BCONPtrWord (fromIntegral p))
+      _ -> panic "mallocStrings:spliceLit"
   spliceLit other = return other
 
   splicePtr (BCOPtrBCO bco) = BCOPtrBCO <$> splice bco
diff --git a/compiler/ghci/RtClosureInspect.hs b/compiler/ghci/RtClosureInspect.hs
index 5b4a10f05e..95c2e37136 100644
--- a/compiler/ghci/RtClosureInspect.hs
+++ b/compiler/ghci/RtClosureInspect.hs
@@ -308,8 +308,10 @@ cPprTerm printers_ = go 0 where
   go prec t = do
     let default_ = Just `liftM` pprTermM go prec t
         mb_customDocs = [pp prec t | pp <- printers] ++ [default_]
-    Just doc <- firstJustM mb_customDocs
-    return$ cparen (prec>app_prec+1) doc
+    mdoc <- firstJustM mb_customDocs
+    case mdoc of
+      Nothing -> panic "cPprTerm"
+      Just doc -> return $ cparen (prec>app_prec+1) doc
 
   firstJustM (mb:mbs) = mb >>= maybe (firstJustM mbs) (return . Just)
   firstJustM [] = return Nothing