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Ryan Scott authored
Summary: #14710 revealed two unfortunate regressions related to kind polymorphism that had crept in in recent GHC releases: 1. While GHC was able to catch illegal uses of kind polymorphism (i.e., if `PolyKinds` wasn't enabled) in limited situations, it wasn't able to catch kind polymorphism of the following form: ```lang=haskell f :: forall a. a -> a f x = const x g where g :: Proxy (x :: a) g = Proxy ``` Note that the variable `a` is being used as a kind variable in the type signature of `g`, but GHC happily accepts it, even without the use of `PolyKinds`. 2. If you have `PolyKinds` (but not `TypeInType`) enabled, then GHC incorrectly accepts the following definition: ```lang=haskell f :: forall k (a :: k). Proxy a f = Proxy ``` Even though `k` is explicitly bound and then later used as a kind variable within the same telescope. This patch fixes these two bugs as follows: 1. Whenever we rename any `HsTyVar`, we check if the following three criteria are met: (a) It's a type variable (b) It's used at the kind level (c) `PolyKinds` is not enabled If so, then we have found an illegal use of kind polymorphism, so throw an error. This check replaces the `checkBadKindBndrs` function, which could only catch illegal uses of kind polymorphism in very limited situations (when the bad kind variable happened to be implicitly quantified in the same type signature). 2. In `extract_hs_tv_bndrs`, we must error if `TypeInType` is not enabled and either of the following criteria are met: (a) An explicitly bound type variable is used in kind position in the body of a `forall` type. (b) An explicitly bound type variable is used in kind position in the kind of a bound type variable in a `forall` type. `extract_hs_tv_bndrs` was checking (a), but not (b). Easily fixed. Test Plan: ./validate Reviewers: goldfire, simonpj, bgamari, hvr Reviewed By: simonpj Subscribers: thomie, carter GHC Trac Issues: #14710 Differential Revision: https://phabricator.haskell.org/D4554
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