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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
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