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Introduction of two new flags, for more precise control over the new pattern match checker's behaviour when reasoning about guards. This is supposed to address #11195 (and maybe more performance bugs related to the NP-Hardness of coverage checking). Expected behaviour: * When `-ffull-guard-reasoning` is on, run the new pattern match checker in its full power * When `-ffull-guard-reasoning` is off (the default), for every match, check a metric to see whether pattern match checking for it has high probability of being non performant (at the the moment we check whether the number of guards is over 20 but I would like to use a more precise measure in the future). If the probability is high: - Oversimplify the guards (less expressive but more performant) and run the checker, and - Issue a warning about the simplification that happened. A new flag `-Wtoo-many-guards/-Wno-too-many-guards` suppresses the warning about the simplification (useful when combined with -Werror). Test Plan: validate Reviewers: goldfire, austin, hvr, bgamari Reviewed By: bgamari Subscribers: mpickering, thomie Differential Revision: https://phabricator.haskell.org/D1676 GHC Trac Issues: #11195
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