[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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Description changed by simonpj:

@@ -31,0 +31,3 @@
+
+ The reason HEAD fails to compile `lens`, but GHC 8.0.1 works, is because
+ the fix for #12220 is in HEAD, but not in 8.0.1.

New description:

 The following compiles with GHC 8.0.1 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.

 The reason HEAD fails to compile `lens`, but GHC 8.0.1 works, is because
 the fix for #12220 is in HEAD, but not in 8.0.1.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:28>
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