[GHC] #11405: Incorrect failure of type-level skolem escape check

[GHC]

#11405: Incorrect failure of type-level skolem escape check
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  8.0.1-rc1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 I've fixed this (rather simple) problem locally, but it uncovered a deeper
 one. Consider

 {{{#!hs
 x :: forall (v :: Levity) (a :: TYPE v). (?callStack :: CallStack) => a
 x = undefined
 }}}

 Looks innocent enough. But disaster ensues with the generated core, which
 is like this:

 {{{
 x = \ @(v :: Levity) @(a :: TYPE v) ($dict :: (?callStack :: CallStack))
 ->
     let $dict' = ... in
     letrec x0 :: a
            x0 = undefined @v @a $dict' in
     x0
 }}}

 The problem here is that `x0` is an abomination -- a levity-polymorphic
 binder, which we have decided are not allowed. This one is harmless (I
 think), but Core Lint doesn't know that.

 Any good ideas of how to proceed?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11405#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler