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

4 files changed, 4 insertions, 4 deletions

diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
index 707e153a8d..a26c565b03 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsNotOn.hs
@@ -1,6 +1,6 @@
 -- Test purpose:
 -- Ensure that not using -Wcompat does not enable its warnings
-
+{-# LANGUAGE NoMonadFailDesugaring #-}
 -- {-# OPTIONS_GHC -Wcompat #-}
 -- {-# OPTIONS_GHC -Wno-compat #-}
 
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
index 777c11cd70..33c26ccbc1 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOff.hs
@@ -1,6 +1,6 @@
 -- Test purpose:
 -- Ensure that using -Wno-compat does not switch on warnings
-
+{-# LANGUAGE NoMonadFailDesugaring #-}
 -- {-# OPTIONS_GHC -Wcompat #-}
 {-# OPTIONS_GHC -Wno-compat #-}
 
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
index 6d67ed039f..7d9e7de4fa 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOn.hs
@@ -1,6 +1,6 @@
 -- Test purpose:
 -- Ensure that -Wcompat switches on the right warnings
-
+{-# LANGUAGE NoMonadFailDesugaring #-}
 {-# OPTIONS_GHC -Wcompat #-}
 -- {-# OPTIONS_GHC -Wno-compat #-}
 
diff --git a/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
index e6a4aa3efb..81df7577e2 100644
--- a/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
+++ b/testsuite/tests/wcompat-warnings/WCompatWarningsOnOff.hs
@@ -1,6 +1,6 @@
 -- Test purpose:
 -- Ensure that -Wno-compat disables a previously set -Wcompat
-
+{-# LANGUAGE NoMonadFailDesugaring #-}
 {-# OPTIONS_GHC -Wcompat #-}
 {-# OPTIONS_GHC -Wno-compat #-}