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Ryan Scott authored
Summary: Previously, `check_type` (which catches illegal uses of unsaturated type synonyms without enabling `LiberalTypeSynonyms`, among other things) always checks for uses of polytypes before anything else. There is a problem with this plan, however: checking for polytypes requires decomposing `forall`s and other invisible arguments, an action which itself expands type synonyms! Therefore, if we have something like: ```lang=haskell type A a = Int type B (a :: Type -> Type) = forall x. x -> x type C = B A ``` Then when checking `B A`, `A` will get expanded to `forall x. x -> x` before `check_type` has an opportunity to realize that `A` is an unsaturated type synonym! This is the root cause of #15954. This patch fixes the issue by moving the case of `check_type` that detects polytypes to be //after// the case that checks for `TyConApp`s. That way, the `TyConApp` case will properly flag things like the unsaturated use of `A` in the example above before we ever attempt to check for polytypes. Test Plan: make test TEST=T15954 Reviewers: simonpj, bgamari, goldfire Reviewed By: simonpj Subscribers: rwbarton, carter GHC Trac Issues: #15954 Differential Revision: https://phabricator.haskell.org/D5402
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