[GHC] #11473: Levity polymorphism checks are inadequate

[GHC]

#11473: Levity polymorphism checks are inadequate
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        Reporter:  simonpj           |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism, TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 This slight variation of Ben's program does produce a garbage result. I
 just added an extra function call to `hello` (and turned on
 `ScopedTypeVariables`).
 {{{
 {-# LANGUAGE PolyKinds, TypeFamilies, MagicHash, DataKinds, TypeInType,
 RankNTypes, ScopedTypeVariables #-}

 import GHC.Exts
 import GHC.Types

 type family Boxed (a :: k) :: *
 type instance Boxed Char# = Char
 type instance Boxed Char  = Char

 class BoxIt (a :: TYPE lev) where
     boxed :: a -> Boxed a

 instance BoxIt Char# where boxed x = C# x
 instance BoxIt Char  where boxed = id

 hello :: forall (lev :: Levity) (a :: TYPE lev). BoxIt a => a -> Boxed a
 hello x = boxed (myid x)
   where myid :: a -> a
         myid y = y
         {-# NOINLINE myid #-}
 {-# NOINLINE hello #-}

 main :: IO ()
 main = do
     print $ boxed 'c'#
     print $ boxed 'c'
     print $ hello 'c'
     print $ hello 'c'#    -- this one prints '\8589966336'
 }}}
 (Interesting note, `hello :: forall foo. forall bar. t` does ''not'' bring
 `bar` into scope in the definition of `hello`. I guess that makes sense.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11473#comment:7>
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