[GHC] #11473: Levity polymorphism checks are inadequate

[GHC]

#11473: Levity polymorphism checks are inadequate
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        Reporter:  simonpj           |                Owner:
            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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Description changed by bgamari:

@@ -2,1 +2,1 @@
- {{{
+ {{{#!hs

New description:

 Ben found
 {{{#!hs
 {-# LANGUAGE PolyKinds, TypeFamilies, MagicHash, DataKinds, TypeInType,
 RankNTypes #-}

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

 main :: IO ()
 main = do
     print $ boxed 'c'#
     print $ boxed 'c'
     print $ hello 'c'
     print $ hello 'c'#
 }}}
 This is plainly wrong because we have a polymorphic function `boxed` that
 is being passed both boxed and unboxed arguments.

 You do get a Lint error with `-dcore-lint`.

 But the original problem is with the type signature for `boxed`.  We
 should never have a levity-polymorphic type to the left of an arrow.  To
 the right yes, but to the left no.   I suppose we could check that in
 `TcValidity`.

 See also #11471

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11473#comment:4>
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