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Empty case expressions have strict semantics so the problem boils down to inhabitation checking for the type of the scrutinee. 3 main functions added: - pmTopNormaliseType_maybe for the normalisation of the scrutinee type - inhabitationCandidates for generating the possible patterns of the appropriate type - checkEmptyCase' to filter out the candidates that give rise to unsatisfiable constraints. See Note [Checking EmptyCase Expressions] in Check and Note [Type normalisation for EmptyCase] in FamInstEnv Test Plan: validate Reviewers: simonpj, goldfire, dfeuer, austin, bgamari Reviewed By: bgamari Subscribers: mpickering, thomie Differential Revision: https://phabricator.haskell.org/D2105 GHC Trac Issues: #10746
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