[GHC] #16244: Couldn't match kind ‘k1’ with ‘k1’

27 Jan 2019

      #16244: Couldn't match kind ‘k1’ with ‘k1’
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.10.1
          Component:  Compiler       |           Version:  8.6.3
  (Type checker)                     |
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 The following program gives a hopelessly confusing error message on GHC
 8.6.3 and HEAD:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE PolyKinds #-}
 module Bug where

 import Data.Kind

 type Const a b = a
 type SameKind (a :: k) (b :: k) = (() :: Constraint)
 class SameKind a b => C (k :: Const Type a) (b :: k)
 }}}
 {{{
 $ /opt/ghc/8.6.3/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:11:18: error:
     • Couldn't match kind ‘k1’ with ‘k1’
     • In the second argument of ‘SameKind’, namely ‘b’
       In the class declaration for ‘C’
    |
 11 | class SameKind a b => C (k :: Const Type a) (b :: k)
    |                  ^
 }}}

 I imagine that the real issue is that `SameKind a b` would force `a :: k`,
 which would be ill-scoped. But figuring that out from this strange error
 message requires a lot of thought.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16244
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