Odd error message due to PolyKinds

[Kosyrev Serge]

Good day folks,

Here is a test case that exposes a certain weakness in PolyKinds
error messaging.  In the following snippet (T W) :: * is malkinded:

> {-# LANGUAGE PolyKinds #-}
> {-# LANGUAGE UnicodeSyntax #-}
> 
> class Eq a ⇒ EqSyn a where
> data T w
> data W f
> 
> x = (undefined ∷ EqSyn w ⇒ T w → ()) (undefined ∷ T W)

8.0.1rc2 yields:

> ../ghc-playground.hs:8:42: error:
>     • Expected kind ‘T W’, but ‘undefined :: T W’ has kind ‘T W’
>     • In the first argument of ‘undefined ::
>                                   EqSyn w => T w -> ()’, namely
>         ‘(undefined :: T W)’
>       In the expression:
>         (undefined :: EqSyn w => T w -> ()) (undefined :: T W)
>       In an equation for ‘main’:
>           main = (undefined :: EqSyn w => T w -> ()) (undefined :: T W)

..which is.. not informative.

With PolyKinds removed:

> ../ghc-playground.hs:7:56: error:
>     • Expecting one more argument to ‘W’
>       Expected a type, but ‘W’ has kind ‘* -> *’
>     • In the first argument of ‘T’, namely ‘W’
>       In an expression type signature: T W
>       In the first argument of ‘undefined ::
>                                   EqSyn w => T w -> ()’, namely
>         ‘(undefined :: T W)’

I'm not sure if this was reported, so I'm sorry for the noise if it was.

-- 
с уважениeм / respectfully,
Косырев Сергей