[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:  8.0.2
       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):

 Ryan, I am very confused here. If I try the code from the ticket
 description, I can't call `foo @Char` so the defined instance is useless.
 Is this intentional? The same happens on GHC 8.0.1 and GHC 7.10.3.

 {{{

 *Foo> :t foo @Char
 foo @Char :: (Char ~ Int => Int) -> Char -> Char
 *Foo> :t foo @Char 5 'a'

 <interactive>:1:1: error:
     • Couldn't match type ‘Char’ with ‘Int’
       Inaccessible code in
         a type expected by the context:
           Char ~ Int => Int
     • In the second argument of ‘foo’, namely ‘5’
       In the expression: foo @Char 5 'a'
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:17>
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