[GHC] #14861: QuantifiedConstraints: Can't use forall'd variable in context

[GHC]

#14861: QuantifiedConstraints: Can't use forall'd variable in context
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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):

 I don't think we're talking about the same thing here. You're talking
 about solving `beta := Int`, but I'm not asking for that! After, if you
 had:

 {{{#!hs
 instance forall p. Show p => Show T
 }}}

 Then that forces GHC to be //parametric// in `p` (so trying to solve `p :=
 Int` would certainly be nonsense).

 Similarly, in:

 {{{#!hs
 instance (forall p. c p => Foldable' f)  -- Yikes: nothing constrains p
        => Foldable' (ECC1 c f) where
 }}}

 You put "Yikes: nothing constrains `p`" here, but that is intentional! The
 whole point is that this constraint must be //parametric// in `p`.
 Therefore, if we have a given constraint of the form `c Int`, then this
 certainly couldn't be used to discharge that quantified constraint.

 However, the code I wrote should work, since the given constraint we have
 is `c p0`—this does not violate the parametricity of `p`, as `p0` is
 simply a skolem.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14861#comment:4>
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