[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 simonpj):

 But the error is at line 17, where we don't know `f` and cannot reduce
 `Apply f a`.  So we really do have a local instance
 {{{
 local instance forall a. Show (Apply f a)
 }}}
 You might argue that we should use it to solve `Show (Apply f Int)` or any
 other instantiation, but we don't. Because we don't expect type functions
 in instance heads.

 (And BTW it's possible that `Show (Apply f Int)` might reduce to `Show
 Bool` and we'd be stuck.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14860#comment:3>
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