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

[GHC]

#15290: QuantifiedConstraints: panic "addTcEvBind NoEvBindsVar"
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        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:                    |
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Comment (by RyanGlScott):

 Ah, my apologies. I interpreted that list of bullet points as three
 separate criteria under which `deriving` would fail, not as a list of
 three conditions which must simultaneously hold.

 In that case, I feel much better about this idea (if this really is the
 only way forward). I've always thought that derived instance methods
 should use `InstanceSigs`, and this gives us a good excuse to do so. (And
 I think certain folks, possibly from Iceland, will find that more
 aesthetically pleasing to look at in `-ddump-deriv` output.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:10>
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