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As the type of a pattern synonym cannot in general be represented by a value of type Type, we cannot use a value `SigSkol (PatSynCtxt n) (Check ty)` to represent the signature of a pattern synonym (this causes incorrect signatures to be printed in error messages). Therefore we now represent it by a value `PatSynSigSkol n` (instead of incorrect signatures we simply print no explicit signature). Furthermore, we rename `PatSynCtxt` to `PatSynBuilderCtxt`, and use `SigSkol (PatSynBuilderCtxt n) (Check ty)` to represent the type of a bidirectional pattern synonym when used in an expression context. Before, this type was represented by a value `SigSkol (PatSynCtxt n) (Check ty)`, which caused incorrect error messages. Also, in `mk_dict_err` of `typecheck\TcErrors.hs` we now distinguish between all enclosing implications and "useful" enclosing implications, for better error messages concerning pattern synonyms. See `Note [Useful implications]`. See the Phabricator page for examples. Reviewers: mpickering, goldfire, simonpj, austin, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1967 GHC Trac Issues: #11667
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