[GHC] #14873: GHC HEAD regression (piResultTy)

[GHC]

#14873: GHC HEAD regression (piResultTy)
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 I think we need these invariants:

 1. If `ty <- tc_hs_type ... exp_kind`, then `typeKind ty == exp_kind`.

 2. If `(ty, ki) <- tc_infer_hs_type ...`, then `typeKind ty == ki`.

 3. If `(ty, ki) <- tc_infer_hs_type ...`, then `zonk ki == ki`.

 4. If `(ty, ki) <- tcTyVar ...`, then `typeKind ty == ki`.

 5. If `(ty, ki) <- tcTyVar ...`, then `zonk ki == ki`.

 All of these appear to be true now, except I'm worried about the `tcTyVar`
 case for `TcTyCon`s -- I ''think'' `TcTyCon` kinds can have unzonked
 metavariables, if the `TcTyCon` comes from the non-CUSK case in
 `kcLHsQTyVars`. Do you also think this is possible? If so, we need to zonk
 the kind and wrap the (knot-tied) type in a reflexive coercion (with
 `mkNakedCastTy`) to fix its kind. Do you agree?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14873#comment:6>
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