[GHC] #15347: QuantifiedConstraints: Implication constraints with type families don't work

[GHC]

#15347: QuantifiedConstraints: Implication constraints with type families don't
work
-------------------------------------+-------------------------------------
        Reporter:  aaronvargo        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by aaronvargo):

 Replying to [comment:16 simonpj]:
 > Really?  What wrong with this?

 {{{
 Not in scope: type variable ‘ft’
 }}}

 The superclass isn't allowed to mention variables which don't appear in
 the head, even if they're already determined by other parameters. The
 solution to this limitation has been "use type families", which leads us
 to this issue.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15347#comment:17>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler