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Simon Peyton Jones authored
Various tickets have revealed bad shortcomings in the typechecking of pattern type synonyms. Discussed a lot in (the latter part of) Trac #11224. This patch fixes the most complex issues: - Both parser and renamer now treat pattern synonyms as an ordinary LHsSigType. Nothing special. Hooray. - tcPatSynSig (now in TcPatSyn) typechecks the signature, and decomposes it into its pieces. See Note [Pattern synonym signatures] - tcCheckPatSyn has had a lot of refactoring. See Note [Checking against a pattern signature] The result is a lot tidier and more comprehensible. Plus, it actually works! NB: this patch doesn't actually address the precise target of #11224, namely "inlining pattern synonym does not preserve semantics". That's an unrelated bug, with a separate patch. ToDo: better documentation in the user manual Test Plan: Validate Reviewers: austin, hvr, goldfire Subscribers: goldfire, mpickering, thomie, simonpj Differential Revision: https://phabricator.haskell.org/D1685 GHC Trac Issues: #11224
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