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Summary: Fix a bug in commit 12eeb9 which permits the following: ``` class C a where type T a b instance C (Maybe a) where type forall a b. T (Maybe a) b = b ``` where instead, the user should write: ``` instance C (Maybe a) where type forall b. T (Maybe a) b = b ``` Update the users guide to discuss scoping of type variables in explicit foralls in type family instances. Test Plan: validate Reviewers: bgamari, goldfire, monoidal Reviewed By: goldfire Subscribers: monoidal, rwbarton, carter GHC Trac Issues: #15828 Differential Revision: https://phabricator.haskell.org/D5283
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