[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):

 > These would be easy to spot -- you just look for a type variable used as
 an argument to another type variable

 I'd say "possible" rather than "easy". You'd have something like
 {{{
 [W] m t1 ~R# m t2
 }}}
 and you want to say "one way to prove this would be to assume some
 additional property of m (the quantified constraint) and, in addition,
 prove `t1 ~R# t2`".

 Wildly ad-hoc, but maybe.  I wouldn't rush to do it.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9123#comment:65>
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