[GHC] #15989: Adding extra quantified constraints leads to resolution failure

[GHC]

#15989: Adding extra quantified constraints leads to resolution failure
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        Reporter:  eror              |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler (Type    |              Version:  8.6.2
  checker)                           |
      Resolution:                    |             Keywords:
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 eror):

 * `( forall x x'. Monad (m x), forall x. Traversable (m x) )` and `(
 forall x x'. Traversable (m x), forall x. Monad (m x) )` both fail on
 `fmap` but not `pure`, which makes it look like the problem is in
 resolving classes that are superclasses of more than one of the
 constraints.
 * Using two type variables in both constraints compiles; it looks like the
 problem arises when the number of quantified type variables is different.
 But `( forall x. Monad (m x), Monad m y )` compiles.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15989#comment:1>
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