[GHC] #15142: GHC HEAD regression: tcTyVarDetails

[GHC]

#15142: GHC HEAD regression: tcTyVarDetails
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Further observations:

 * `MultiParamTypeClasses` is also important to triggering this, since this
 panics:

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

   import Data.Kind

   class C (a :: Type) (b :: k) where
     type T b
   }}}

   But not this:

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

   import Data.Kind

   class C (b :: k) where
     type T b
   }}}
 * The order of arguments to `C`, as well as the exact kind signatures
 given to them, also appears to be important, as none of the following
 panics:

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

   import Data.Kind

   class C1 (b :: k) (a :: Type) where
     type T1 b

   class C2 b (a :: Type) where
     type T2 b

   class C3 b a where
     type T3 b

   class C4 (b :: k) a where
     type T4 b

   class C5 a (b :: k) where
     type T5 b

   class C6 (a :: Type) b where
     type T6 b

   class C7 a b where
     type T7 b
   }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15142#comment:3>
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