[GHC] #12882: Unexpected constraint when using ExistentialQuantification

[GHC]

#12882: Unexpected constraint when using ExistentialQuantification
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:  invalid           |             Keywords:
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 rwbarton):

 Well there is no problem if you write `data` instead of `newtype`, above.
 But of course the `Foo a` dictionary must be stored within an `ImplicitFoo
 a`, so that it can provide the implementation of `foo` at runtime. You
 cannot think of the constraint `Foo a` as a box to be checked off to
 satisfy some bureaucratic type checker.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12882#comment:7>
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