[GHC] #15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  9123, 14883
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Yes, I agree.  I'll stop the panic (by refraining from using quantified
 constraints if I have no place to put the evidence) but that will make GHC
 reject `deriving` for
 * A higher rank method
 * That makes essential use of a quantified constraint
 * Under a forall

 I don't see how to avoid that restriction.   If anyone else does, please
 say so!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:7>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler