[GHC] #14584: Core Lint error

[GHC]

#14584: Core Lint error
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType,
                                     |  DeferredTypeErrors
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 simonpj):

 ...or not.   Monoidal's example is amazing. Here's the constraint we get
 (before simplifying it):
 {{{
 simplifyTop {
   wanted =  WC {wc_impl =
                   Implic {
                     TcLevel = 2
                     Skolems = k_aGF[sk:2] (m_aGG[sk:2] :: k_aGF[sk:2])
                     No-eqs = False
                     Status = Unsolved
                     Given =
                     Wanted =
                       WC {wc_simple =
                             [WD] hole{aGP} {0}:: ((m_aGG[sk:2] |> {aGN})
                                                   -> (m_aGG[sk:2] |>
 {aGN}) :: *)
                                                  GHC.Prim.~# (() :: *)
 (CNonCanonical)
                           wc_impl =
                             Implic {
                               TcLevel = 3
                               Skolems =
                               No-eqs = False
                               Status = Unsolved
                               Given =
                               Wanted =
                                 WC {wc_simple =
                                       [D] _ {0}:: (m_aGG[sk:2] |> {aGN})
                                                   -> (m_aGG[sk:2] |>
 {aGN}) (CHoleCan: TypeHole(_))
                                       [WD] hole{aGN} {0}:: (k_aGF[sk:2] ::
 *)
                                                            GHC.Prim.~# (*
 :: *) (CNonCanonical)}
                               Binds = EvBindsVar<aGO>
                               Needed = []
                               the inferred type of <expression>_02y ::
 w_aGJ[tau:3] }}
                     Binds = EvBindsVar<aGQ>
                     Needed = []
                     the type signature for:
                       f :: forall k (m :: k). () }}
 }}}
 Notice that the coercion `{aGN}` is a Wanted inside the inner implication
 constraint, but is used a constraint (for `{aGP}`) outside that
 implication.  No amount of floating is going to put that right!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14584#comment:6>
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