[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by mpickering):

 I don't think it is clear from this ticket how this code was expected to
 work in the past.

 In previous versions, it was possible to use type signatures like this in
 order to state that only `Int` could provide a different definition from
 the default definition. So in Ryan's example, `instance Foo Char` would
 compile with the default definition for `foo`. This always seemed like a
 bug too me.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:5>
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