[GHC] #12703: Expand Backpack's signature matching relation beyond definitional equality

[GHC]

#12703: Expand Backpack's signature matching relation beyond definitional equality
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        Reporter:  ezyang            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ezyang):

 One of the main technical challenges is extending our declaration merging
 operation to handle any such subtyping relation.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12703#comment:2>
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