#15412: "Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by monoidal): The failing code is obtained by replacing `Constraint` by `C`: {{{ {-# Language DataKinds, TypeInType, TypeFamilies #-} import Data.Kind newtype I a = I a type C = Constraint type family UnitC :: C where UnitC = () instance UnitC => Functor I where fmap = undefined }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler