[GHC] #10493: Inaccessible code might be accessible with newtypes and Coercible

[GHC]

#10493: Inaccessible code might be accessible with newtypes and Coercible
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        Reporter:  goldfire          |                   Owner:  goldfire
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 It would be nice to combine `typesCantMatch` with the apartness check, but
 there's a key difference: `typesCantMatch` is about representational
 equality, but apartness is about nominal equality.

 In a perfect world, we would combine `typesCantMatch`, `tcMatchTys`,
 `tcUnifyTys`, and even `eqType`. But these are all subtly different, and
 the world isn't perfect. So, I propose to leave this for now.

 I'll update the comments and explain why we can't merge `typesCantMatch`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10493#comment:3>
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