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Previously when the checker encountered an unsatisfiable term of type context it would return an empty initial uncovered set. This caused all pattern matches in the context to be reported as redudant. This is arguably correct behaviour as they will never be reached but it is better to recover and provide accurate warnings for these cases to avoid error cascades. It would perhaps be better to report an error to the user about an inacessible branch but this is certainly better than many confusing redundant match warnings. Reviewers: gkaracha, austin, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D3064
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