[GHC] #11715: There's something fishy going on with Constraint

[GHC]

#11715: There's something fishy going on with Constraint
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |
      Resolution:                    |             Keywords:
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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Changes (by adamgundry):

 * cc: adamgundry (added)


Comment:

 The current behaviour definitely sounds suspicious. I notice that 8.0 RC2
 accepts the following:
 {{{#!hs
 {-# LANGUAGE TypeInType, TypeFamilies #-}

 import Data.Kind

 type family F (a :: *) :: *
 type instance F Type       = Int
 type instance F Constraint = Bool
 }}}
 But surely if `Type ~ Constraint` in Core, and this definition is
 accepted, then Core is unsound? I'm not sure if this can be exploited in
 surface Haskell, but Ben's example makes me think it might.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11715#comment:6>
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