[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):

 (Why do I want to use a constraint like this in the first place? I
 discovered this while trying to write something like

 {{{
 ( forall x x'. x ~ x' => MonadReader (Env x) (m x')
 , forall x. MonadWriter Log (m x)
 )
 }}}

 where the two type variables and equality constraint are there to work
 around #15351.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15989#comment:2>
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