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

5 Aug 2016

      #12466: Typechecker regression: Inaccessible code in a type expected by the context
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:  8.2.1
          Component:  Compiler       |           Version:  8.1
  (Type checker)                     |
           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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 The following compiles with GHC 7.10.3 and earlier, but not with GHC HEAD:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 class Foo a where
   foo :: (a ~ Int => Int) -> a -> a
   foo _ a2 = a2

 instance Foo Char
 }}}

 {{{
 $ /opt/ghc/head/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:9:10: error:
     • Couldn't match type ‘Char’ with ‘Int’
       Inaccessible code in
         a type expected by the context:
           Char ~ Int => Int
     • In the expression: Bug.$dmfoo @Char
       In an equation for ‘foo’: foo = Bug.$dmfoo @Char
       In the instance declaration for ‘Foo Char’
 }}}

 This causes `lens-4.14` to [https://github.com/ekmett/lens/issues/667 fail
 to build] with GHC HEAD.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12466
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