[GHC] #9151: Recursive default associated types don't kind-generalize properly

[GHC]

#9151: Recursive default associated types don't kind-generalize properly
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by archblob):

 I have played with the test case a little to see if I can reduce it, and
 to see when exactly the error happens.

 Here is what I have:
 {{{#!haskell
 {-# LANGUAGE PolyKinds, TypeFamilies, UndecidableInstances #-}

 module Bug where

 class PEnum (k :: a) where
   type ToEnum (x :: a) :: *
   type ToEnum x = TEHelper

 type TEHelper = ToEnum Int
 }}}

 That fails as the test case in the ticket, and this one passes:
 {{{#!haskell
 {-# LANGUAGE PolyKinds, TypeFamilies, UndecidableInstances #-}

 module Bug where

 class PEnum (k :: a) where
   type ToEnum (x :: b) :: *
   type ToEnum x = TEHelper

 type TEHelper = ToEnum Int
 }}}

 Also this is the output from {{{-ddump-tc-trace}}} :

 {{{
 rn12
 rn13
 Tc2 (src)
 Tc3
 kcTyClGroup
   module Bug
   class PEnum (k :: a) where
     type family ToEnum (x :: a) :: *
     type instance ToEnum x = TEHelper
   type TEHelper = ToEnum Int
 env2 [(a, Type variable ‘a’ = a)]
 env2 []
 kcTyClGroup: initial kinds
   [(PEnum, AThing a -> Constraint), (ToEnum, AThing a -> *)]
 env2 []
 kcd1 TEHelper []
 tc_lhs_type:
   ToEnum Int
   Expected kind ‘k_av5’
 tc_lhs_type:
   ToEnum
   Expected kind ‘k_av6’
 lk1 ToEnum
 lk2 ToEnum AThing a -> *
 writeMetaTyVar k_av6 := a -> *
 tc_lhs_type:
   Int
   The first argument of ‘ToEnum’ should have kind ‘a’
 lk1 Int
 lk2 Int Type constructor ‘Int’
 checkExpectedKind
   Int
   *
   a
 checkExpectedKind 1
   Int
   *
   a
   ([(k0, 1)], [(av4, a)])
   ([(k0, 1)], [(av4, a)])
 Adding error:
   Bug.hs:9:24:
       The first argument of ‘ToEnum’ should have kind ‘a’,
         but ‘Int’ has kind ‘*’
       In the type ‘ToEnum Int’
       In the type declaration for ‘TEHelper’
 tryTc/recoverM recovering from IOEnv failure

 Bug.hs:9:24:
     The first argument of ‘ToEnum’ should have kind ‘a’,
       but ‘Int’ has kind ‘*’
     In the type ‘ToEnum Int’
     In the type declaration for ‘TEHelper’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9151#comment:1>
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