[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 It's simple. Consider
 {{{
 class C v where
   type T a b v cd
 }}}
 In any instance declaration, the type in the `v` position of the `type`
 instance must be the same as in the instance header:
 {{{
 instance ... => C ty where
   type T p q ty r s = <rhs>
 }}}
 I think we do (or at least should) also insist that `p`, `q`, `r`, `s` are
 distinct type variables, otherwise the type instance is more specific than
 the class instance, but that's a separate matter.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:8>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler