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