[GHC] #11203: Kind inference with SigTvs is wrong

[GHC]

#11203: Kind inference with SigTvs is wrong
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider

 {{{#!hs
 data SameKind :: k -> k -> *

 data Q (a :: k1) (b :: k2) c = MkQ (SameKind a b)
 }}}

 This code should be rejected. Yet it is accepted. The problem is that,
 when kind-checking a datatype declaration without a CUSK, GHC uses
 `SigTv`s for user-written kind variables. `SigTv`s are allowed to unify
 with other `SigTv`s, leading to incorrect behavior here.

 The motivating scenario is this:

 {{{#!hs
 data T (a :: k1) x = MkT (S a ())
 data S (b :: k2) y = MkS (T b ())
 }}}

 This program should be accepted. Neither type has a CUSK and therefore is
 not generalized before kind-checking the group. GHC must then unify `k1`
 and `k2`. If they were skolems, this unification would fail and this pair
 of definitions would be rejected.

 This ticket is identical in spirit to #9201, but that example has a CUSK
 and thus works. The bug can happen only when there is no CUSK.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11203>
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