[GHC] #15122: GHC HEAD typechecker regression

[GHC]

#15122: GHC HEAD typechecker regression
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        Reporter:  fmixing           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 OK, even smaller now:

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

 import Data.Kind
 import Data.Proxy

 data IsStar (a :: k) where
   IsStar :: IsStar (a :: *)

 type family F (a :: k) :: k

 foo :: (F a ~ F b) => IsStar a -> Proxy b
                    -> Proxy (F a) -> Proxy (F b)
 foo IsStar _ p = p
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15122#comment:8>
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