[GHC] #14860: QuantifiedConstraints: Can't quantify constraint involving type family

[GHC]

#14860: QuantifiedConstraints: Can't quantify constraint involving type family
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:  wontfix           |  QuantifiedConstraints wipT2893
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 Iceland_jack):

 These problems were my original motivation for #14822, if `f` has a
 singleton we can witness `Show (Apply f a)` without changing the
 definition of `Apply`

 {{{#!hs
 wit :: forall f a. SingI f :- Show (Apply f a)
 wit = Sub
   (case sing :: Sing f of
     SingProxy -> Dict)
 }}}

 If we had a way of turning `(a :- b)` into `(a => b)` I think this would
 solve Ryan's problem?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:14>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler