[GHC] #12201: Wrong instance selection with overlapping instances and local bindings

17 Jun 2016


      #12201: Wrong instance selection with overlapping instances and local bindings
-------------------------------------+-------------------------------------
           Reporter:  kanetw         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 test and test2 should have the same type, but it seems like the {-#
 OVERLAPPING #-} instance is chosen prematurely.
 {{{#!hs
 {-# LANGUAGE NoMonomorphismRestriction, FlexibleContexts,
 FlexibleInstances #-}
 module Bug where

 class A a where
 class A a => B a where

 data Foo s a
 instance {-# OVERLAPPING #-} A (Foo Int Bool)
 instance A (Foo s a)
 instance B (Foo s a)

 bar :: (A a, B a) => a
 bar = undefined

 helper :: Foo s Bool -> s -> Int
 helper = undefined

 {-
   *Bug> :t test
   test :: A (Foo s Bool) => s -> Int
 -}
 test = helper bar

 {-
   *Bug> :t test2
   test2 :: s -> Int
 -}
 test2 = let foo = bar in helper foo
 }}}

 Compare with 7.10.3:
 {{{
 *Bug> :t test
 test :: A (Foo s Bool) => s -> Int
 *Bug> :t test2
 test2 :: A (Foo s Bool) => s -> Int
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12201
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