[GHC] #14968: QuantifiedConstraints: Can't be RHS of type family instances

[GHC]

#14968: QuantifiedConstraints: Can't be RHS of type family instances
-------------------------------------+-------------------------------------
        Reporter:  josef             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9269             |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 I'm lost. Is there some aspect of quantified constraints that would make
 this OK:

 {{{#!hs
 type instance Functors (t ': ts) = (forall f. Functor f => Functor (t f),
 Functors ts)
 }}}

 But not this (from
 https://ghc.haskell.org/trac/ghc/ticket/9269#comment:1)?

 {{{#!hs
 type instance Foo True = forall a. a -> a -> a
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14968#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler