[GHC] #13347: Abstract classes in hs-boot should not be treated as injective

[GHC]

#13347: Abstract classes in hs-boot should not be treated as injective
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           Reporter:  ezyang         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           Keywords:  hs-boot        |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 They could be implemented by a newtype (see `mkClass`), making them non-
 injective.

 Fortunately, this doesn't seem to cause any unsoundness, because the
 *constructor* for the newtype corresponding to a class is never in scope,
 so we are never allowed to unwrap "class newtypes". Phew!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13347>
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