[GHC] #11471: Note [The kind invariant] in TyCoRep needs to be updated

[GHC]

#11471: Note [The kind invariant] in TyCoRep needs to be updated
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        Reporter:  bgamari           |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.3
  checker)                           |
      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:                    |
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Comment (by simonpj):

 I wrote this code to try to get a seg-fault or something:
 {{{
 import GHC.Exts
 import Data.Proxy

 type family F a :: k

 type instance F Int = Int#

 f :: Proxy a -> F a -> F a
 f _ x = x

 bad = f (undefined :: Proxy Int#) 3#
 }}}
 But instead I got some truly bizarre error messages
 {{{
 Foo.hs:15:10: error:
     • Expected kind ‘Proxy Int#’,
         but ‘undefined :: Proxy Int#’ has kind ‘Proxy Int#’
     • In the first argument of ‘f’, namely ‘(undefined :: Proxy Int#)’
       In the expression: f (undefined :: Proxy Int#) 3#
       In an equation for ‘bad’: bad = f (undefined :: Proxy Int#) 3#

 Foo.hs:15:35: error:
     • Couldn't match a lifted type with an unlifted type
       When matching types
         F Int# :: *
         Int# :: #
     • In the second argument of ‘f’, namely ‘3#’
       In the expression: f (undefined :: Proxy Int#) 3#
       In an equation for ‘bad’: bad = f (undefined :: Proxy Int#) 3#
 }}}
 We get a bit more of a clue with `-fprint-explicit-kinds`
 {{{
 Foo.hs:15:10: error:
     • Expected kind ‘Proxy * Int#’,
         but ‘undefined :: Proxy Int#’ has kind ‘Proxy # Int#’
     • In the first argument of ‘f’, namely ‘(undefined :: Proxy Int#)’
       In the expression: f (undefined :: Proxy Int#) 3#
       In an equation for ‘bad’: bad = f (undefined :: Proxy Int#) 3#

 Foo.hs:15:35: error:
     • Couldn't match a lifted type with an unlifted type
       When matching types
         F * Int# :: *
         Int# :: #
     • In the second argument of ‘f’, namely ‘3#’
       In the expression: f (undefined :: Proxy Int#) 3#
       In an equation for ‘bad’: bad = f (undefined :: Proxy Int#) 3#
 }}}
 but clearly life is not good here.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11471#comment:2>
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