[GHC] #13140: Handle subtyping relation for roles in Backpack

[GHC]

#13140: Handle subtyping relation for roles in Backpack
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        Reporter:  ezyang            |                Owner:  ezyang
            Type:  feature request   |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:  backpack hs-
      Resolution:                    |  boot
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3123
       Wiki Page:                    |
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Comment (by goldfire):

 The Right Thing to do is to go back in time and erase the idea of roles
 from our brains... and reimplement `GeneralizedNewtypeDeriving` in terms
 of a generated blob of source code that gets type-checked (instead of the
 magical implementation it used to have).

 Barring that, I can't find an argument against a new abstract role other
 than to harrumph as loudly as I can.

 Harrumph!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13140#comment:9>
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