• Ryan Scott's avatar
    Fix #14710 with more validity checks during renaming · 447d1264
    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
    447d1264
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