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

[GHC]

#14860: QuantifiedConstraints: Can't quantify constraint involving type family
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  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:                    |
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Comment (by RyanGlScott):

 In response to goldfire's question, let me ask an analogous one.
 Currently, we allow this:

 {{{#!hs
 instance (forall xx. c (Free c xx)) => Monad (Free c) where
     Free f >>= g = f g
 }}}

 Also, assume there is no `Show (Free Show xx)` instance. Now suppose we
 are given `(forall xx. c (Free c xx))`, and we want to prove `Show (Free
 Show xx)`. In considering the wanted `Show (Free Show xx)`, we have two
 choices of how to proceed:

 1. Use the given and succeed.
 2. Attempt to resolve the `Show (Free Show xx)` instance and fail.

 The exact same problem seems to arise here, so why isn't this disallowed?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:7>
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