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

[GHC]

#14861: QuantifiedConstraints: Can't use forall'd variable in context
-------------------------------------+-------------------------------------
        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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 If I have
 {{{
 instance forall p. Show p => Show T
 }}}
 then when I invoke the instance declaration I get a `Show beta` constraint
 with absolutely nothing telling the compiler what `beta` should be.  Yes,
 it could be solved with `beta := Int` or a variety of other things. But
 GHC never guesses.

 Same here.  You have
 {{{
 instance (forall p. c p => Foldable' f)  -- Yikes: nothing constrains p
        => Foldable' (ECC1 c f) where
 }}}
 When the quantified constraint fires, to solve `Foldable' f`, it generates
 `c beta` with no constraints whatsoever on `beta`.  The fact that we have
 in scope `c p` does not help, because it requires quessing that `beta :=
 p`.  Sorry!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14861#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler