• Ryan Scott's avatar
    Don't quantify implicit type variables when quoting type signatures in TH · 729a5e45
    Ryan Scott authored
    Summary:
    A bug was introduced in GHC 8.0 in which Template Haskell-quoted type
    signatures would quantify _all_ their type variables, even the implicit ones.
    This would cause splices like this:
    
    ```
    $([d| idProxy :: forall proxy (a :: k). proxy a -> proxy a
          idProxy x = x
       |])
    ```
    
    To splice back in something that was slightly different:
    
    ```
    idProxy :: forall k proxy (a :: k). proxy a -> proxy a
    idProxy x = x
    ```
    
    Notice that the kind variable `k` is now explicitly quantified! What's worse,
    this now requires the `TypeInType` extension to be enabled.
    
    This changes the behavior of Template Haskell quoting to never explicitly
    quantify type variables which are implicitly quantified in the source.
    
    There are some other places where this behavior pops up too, including
    class methods, type ascriptions, `SPECIALIZE` pragmas, foreign imports,
    and pattern synonynms (#13018), so I fixed those too.
    
    Fixes #13018 and #13123.
    
    Test Plan: ./validate
    
    Reviewers: simonpj, goldfire, austin, bgamari
    
    Reviewed By: simonpj, goldfire
    
    Subscribers: simonpj, mpickering, thomie
    
    Differential Revision: https://phabricator.haskell.org/D2974
    
    GHC Trac Issues: #13018, #13123
    729a5e45
all.T 15.8 KB