[GHC] #10340: Type inference regression with Any

21 Apr 2015

      #10340: Type inference regression with Any
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              Reporter:  akio        |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
  (Type checker)                     |  Operating System:  Unknown/Multiple
              Keywords:              |   Type of failure:  GHC rejects
          Architecture:              |  valid program
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 The following module typechecks with GHC 7.8.3, but GHC 7.10.1 gives an
 error:

 {{{#!hs
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE FlexibleInstances #-}

 module Foo where

 import GHC.Exts (Any)

 class MonadState s m where
   get :: m s

 newtype State s a = State (s -> (s, a))

 instance MonadState s (State s) where
   get = State $ \s -> (s, s)

 foo :: State Any Any
 foo = get
 }}}

 {{{
 [1 of 1] Compiling Foo              ( any.hs, any.o )

 any.hs:15:7:
     No instance for (MonadState Any (State Any))
       arising from a use of ‘get’
     In the expression: get
     In an equation for ‘foo’: foo = get
 }}}

 If I replace the uses of `Any` with `Int`, the problem goes away.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10340
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