[GHC] #15639: Surprising failure combining QuantifiedConstraints with Coercible

[GHC]

#15639: Surprising failure combining QuantifiedConstraints with Coercible
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
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 simonpj):

 Can you explain more about this example?  I think that it is important
 that

 * You give a role signature for `Yeah`.  (What is the inferred signature?)
 * You export the type constructor for `Yeah` but not the data constructor.

 But I'm a bit lost about what you expect to happen.  Can you amplify?  Why
 do you think it should typecheck? It'd probably help to give the
 declaration for `Coercion` too, for those of us who do not use it daily.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15639#comment:5>
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