[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 goldfire):

 The current behavior was introduced in
 b612da667fe8fa5277fc78e972a86d4b35f98364

 That commit fixes some impredicativity bug but also rewrites GND to use
 type application. However, the change to GND also has it work with
 polytypes instead of its previous behavior, using monotypes. It sounds
 like we want to go to the previous behavior.

 As for Ryan's question: try passing both type arguments to `coerce`. As
 explained in the visible type application paper, type signatures on
 expressions are ''deeply skolemized'', which is causing havoc for you
 here. I think including the second type parameter to `coerce` will solve
 the problem.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15290#comment:16>
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