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Summary: GHC doesn't know how to handle type families that appear in class instances. Unfortunately, GHC didn't reject instances where type families appear in //kinds//, leading to #15515. This is easily rectified by calling `checkValidTypePat` on all arguments to a class in an instance (and not just the type arguments). Test Plan: make test TEST=T15515 Reviewers: bgamari, goldfire, simonpj Reviewed By: simonpj Subscribers: simonpj, rwbarton, carter GHC Trac Issues: #15515 Differential Revision: https://phabricator.haskell.org/D5068
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