[GHC] #9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving

[GHC]

#9123: Emit quantified Coercible constraints in GeneralizedNewtypeDeriving
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        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler (Type    |              Version:  7.8.2
  checker)                           |             Keywords:  Roles,
      Resolution:                    |  QuantifiedConstraints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:  quantified-
                                     |  constraints/T9123{,a}
      Blocked By:  15290             |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Instead, we could do it in TcDerivInfer by anticipating the need for
 role-based implication constraint and then providing it.

 `TcDerivInfer` works as follows:

 * Generate constraints
 * Simplify them
 * Decide what to quantify over

 I don't see where in that path we would add "anticipate the need for role-
 based implication constrains and provide it".

 Anyway I think we have bigger fish to fry!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:67>
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