[GHC] #8154: Possible bug in open type familes: Conflicting (a->a) and (a->a->a) instances

[GHC]

#8154: Possible bug in open type familes: Conflicting (a->a) and (a->a->a)
instances
----------------------------+----------------------------------------------
       Reporter:  nh2       |             Owner:
           Type:  bug       |            Status:  new
       Priority:  normal    |         Milestone:  7.8.1
      Component:  Template  |           Version:  7.7
  Haskell                   |  Operating System:  Unknown/Multiple
       Keywords:  type      |   Type of failure:  GHC rejects valid program
  families                  |         Test Case:
   Architecture:            |          Blocking:
  Unknown/Multiple          |
     Difficulty:  Unknown   |
     Blocked By:            |
Related Tickets:            |
----------------------------+----------------------------------------------
 {{{
 {-# LANGUAGE TypeFamilies #-}
 module Test where

 type family BoundsOf x

 type instance BoundsOf (a->a) = Int
 type instance BoundsOf (a->a->a) = (Int,Int)
 }}}

 This worked with GHC 7.6, but not with 7.8 HEAD (currently at 6cc7d3f).

 To check:
 {{{
 wget
 https://gist.github.com/nh2/6302087/raw/8167e7a1c8613aa384c2e8ca2f4ea9ade8745dc1/ghc-7.7-type-a-a-a-families.hs
 ghci ghc-7.7-type-a-a-a-families.hs  # 7.6, all fine
 ghci ghc-7.7-type-a-a-a-families.hs  # 7.7, breaks
 }}}

 On #ghc, we don't really understand whether this is the right thing to
 happen or not.

 {{{
 <rwbarton> see
 http://comments.gmane.org/gmane.comp.lang.haskell.glasgow.user/23734
 <carter> c_wraith: im not sure why thats NOT working for open type familes
 too though
 <carter> a->a and a->a->a don't overlap...
 <rwbarton> ah it's in that thread. "Open (normal, old-fashioned) type
 families are essentially unchanged. In particular, coincident overlap and
 non-linear patterns *are* allowed.  The overlap check between open type
 family instances now does a unification without an "occurs check" to mark
 (x, x) and ([y], y) as overlapping, as necessary for type soundness."
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8154>
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