[GHC] #15862: Panic with promoted types that Typeable doesn't support (was: Typeable panic with promoted rank-2 kind (initDs))

[GHC]

#15862: Panic with promoted types that Typeable doesn't support
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:  Typeable
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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Changes (by RyanGlScott):

 * keywords:  Typeable, ImpredicativeTypes => Typeable


Comment:

 It turns out that this problem is more general than just this one example
 (which happens to use `ImpredicativeTypes`). Here is //another// program
 which triggers the same panic:

 {{{#!hs
 {-# LANGUAGE ExistentialQuantification #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug where

 import Data.Kind
 import Type.Reflection

 type family F a
 type instance F Int = Type

 data Bar = forall (a :: F Int). MkBar a

 bar :: TypeRep (MkBar True)
 bar = typeRep
 }}}


 This time, the culprit is likely the fact that `MkBar True` contains a
 cast/coercion somewhere, which is another thing that `Typeable` doesn't
 support.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15862#comment:4>
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