[GHC] #2677: Detection of TF instance conflict depends on instance order

[GHC]

#2677: Detection of TF instance conflict depends on instance order
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    Reporter:  reinerp                  |        Owner:  chak       
        Type:  bug                      |       Status:  closed     
    Priority:  normal                   |    Milestone:  6.12 branch
   Component:  Compiler (Type checker)  |      Version:  6.10.3     
    Severity:  critical                 |   Resolution:  fixed      
    Keywords:  TF instance conflict     |   Difficulty:  Unknown    
    Testcase:  T2677                    |           Os:  Linux      
Architecture:  x86_64 (amd64)           |  
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Changes (by dorchard):

  * version:  6.9 => 6.10.3
  * os:  Unknown/Multiple => Linux
  * architecture:  Unknown/Multiple => x86_64 (amd64)
  * severity:  normal => critical

Comment:

 Just discovered this bug today but glad to see it has already been picked
 up.
 The bug applies to data families too. You can do some pretty whacked out
 stuff with this bug, like launching missiles ;)

 {{{
 {-# LANGUAGE TypeFamilies #-}

 -- MkFoo is a constructor on pure functions thereore "safe" :)

 data family Foo a
 data instance Foo a = MkFoo (Int -> Int)

 unBox :: Foo a -> (Int -> Int)
 unBox (MkFoo x) = x

 -- Nasty person adds this code in a module

 launchMissiles :: IO ()
 launchMissiles = print ("Launching missiles...")

 data instance Foo Int = MakeLauncher (IO())
 launch = (unBox (MakeLauncher launchMissiles)) 42
 }}}

 launch segfaults, but about 1 in 3 times (depending on the memory layout)
 the missiles are launched!

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2677#comment:8>
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