[GHC] #12174: Recursive use of type-in-type results in infinite loop

[GHC]

#12174: Recursive use of type-in-type results in infinite loop
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           Reporter:  ezyang         |             Owner:  goldfire
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           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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 Typechecking this module results in an infinite loop:

 {{{
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE TypeInType #-}
 module X where

 data V a
 data T = forall (a :: S). MkT (V a)
 data S = forall (a :: T). MkS (V a)
 }}}

 There's a mutually recursive reference so it should be rejected but maybe
 the check is not happening early enough (or the existing check is only for
 self-reference.)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12174>
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