[GHC] #12881: GHC 8.0.2 regression involving OVERLAP annotations

26 Nov 2016

      #12881: GHC 8.0.2 regression involving OVERLAP annotations
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           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.0.2-rc1
           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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 `quickcheck-combinators-0.0.1` fails to build with GHC 8.0.2-rc1 (but does
 build with GHC 8.0.1) due to this issue. Here is a simplified example:

 {{{#!hs
 {-# LANGUAGE FlexibleInstances #-}
 module Bug where

 class Arbitrary a where
   shrink :: a -> [a]
   shrink _ = []

 instance Arbitrary a
 instance Arbitrary Int
 }}}

 Is this expected? If so, we should make a note of this in the 8.0.2
 release notes.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12881
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