[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 goldfire):

 I'm struggling with this one, worried about order-dependence. Suppose
 `Apply f Int` is `Bool`. Now suppose we are given `forall a. Show (Apply f
 a)` and want to prove `Show (Apply f Int)`. In considering the wanted
 `Show (Apply f Int)`, we have two choices of how to proceed:

 1. Use the given and succeed.

 2. Reduce `Apply f Int` to `Bool` and fail.

 The problem is that it seems challenging to guarantee that we don't do
 (2). Simon's answer appears to be to simply disallow the constraint in the
 first place.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:6>
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