[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):

 Replying to [comment:10 simonpj]:
 > Re comment:7 there are no type functions are there.  It's the function
 in the constraint head that is the nub of the problem.

 Again... so what? I'm trying to demonstrate why hypothetically allowing
 type families in quantified constraints would be no different than
 allowing type variables heading instances in quantified constraints. So
 far, the only rebuttal you've given me is "but we don't allow type
 families in instances", which is circular reasoning. So I'm still left
 wondering what exactly the substantive difference between these two
 scenarios is.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:11>
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