1. 14 Oct, 2016 1 commit
    • Ryan Scott's avatar
      Make error when deriving an instance for a typeclass less misleading · d5a4e49d
      Ryan Scott authored
      Before, when you attempted to derive an instance for a typeclass,
      e.g.,
      
      ```
      class C1 (a :: Constraint) where
      class C2 where
      
      deriving instance C1 C2
      ```
      
      GHC would complain that `C2`'s data constructors aren't in scope. But
      that
      makes no sense, since typeclasses don't have constructors! By refining
      the
      checks that GHC performs when deriving, we can make the error message a
      little more sensible.
      
      This also cleans up a related `DeriveAnyClass` infelicity. Before, you
      wouldn't have been able to compile code like this:
      
      ```
      import System.IO (Handle)
      class C a
      deriving instance C Handle
      ```
      
      Since GHC was requiring that all data constructors of `Handle` be in
      scope. But `DeriveAnyClass` doesn't even generate code that mentions
      any data constructors, so this requirement is silly!
      
      Fixes #11509.
      
      Test Plan: make test TEST=T11509
      
      Reviewers: simonpj, austin, bgamari
      
      Reviewed By: simonpj, bgamari
      
      Subscribers: thomie, simonpj
      
      Differential Revision: https://phabricator.haskell.org/D2558
      
      GHC Trac Issues: #11509
      d5a4e49d