-
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