"Instance head is not headed by a class" when `Constraint` replaced with `type C = Constraint`
{-# 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 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>
Trac metadata
Trac field | Value |
---|---|
Version | 8.4.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |