[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:                    |
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Comment (by RyanGlScott):

 Comparing the results of `-ddump-tc-trace` on this program:

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

 import Data.Kind

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

 On GHC 8.4.2, we have:

 {{{
 kcTyClGroup: initial kinds
   [r1xu :-> ATcTyCon C :: forall k. * -> k -> Constraint,
    r1xA :-> ATcTyCon T :: forall k. k -> *]
 }}}

 But on GHC HEAD, we have:

 {{{
 kcTyClGroup: initial kinds
   C :: forall k. * -> k -> Constraint
   T :: forall (k :: k_a1zm[tau:1]) (co :: k_a1zm[tau:1]
                                           GHC.Prim.~# *).
        (k |> {co_a1zq}) -> *
 }}}

 And indeed, `tcTyVarDetails` appears to be panicking on `co`. But I
 haven't the foggiest idea of what it's doing there...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15142#comment:4>
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