#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 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# Language DataKinds, TypeInType, TypeFamilies #-} import Data.Kind newtype I a = I a type C = Constraint type family UnitC :: Constraint where UnitC = () instance UnitC => Functor I where fmap = undefined }}} this works fine, but if I try to use the constraint synonym `C` (`UnitC :: C where`) fails with "Instance head is not headed by a class" which has '''no''' Google hits outside of the [https://hackage.haskell.org/package/ghc-8.4.1/docs/src/TcValidity.html#check... compiler] {{{ $ ghci -ignore-dot-ghci hs/249.hs GHCi, version 8.5.20180128: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( hs/249.hs, interpreted ) hs/249.hs:13:10: error: • Instance head is not headed by a class • In the instance declaration for ‘Functor I’ | 13 | instance UnitC => Functor I where | ^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. Prelude> }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15412 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler