[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       |           Version:  8.5
  (Type checker)                     |
           Keywords:  TypeInType,    |  Operating System:  Unknown/Multiple
  TypeFamilies                       |
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash or panic
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 This regression prevents the `generic-lens` library from building. Here is
 a minimized test case:

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

 import Data.Kind

 class ListTuple (tuple :: Type) (as :: [(k, Type)]) where
   type ListToTuple as :: Type
 }}}

 On GHC 8.4.2, this compiles, but on GHC HEAD, it panics:

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 Bug.hs[1 of 1] Compiling Bug
 ( Bug.hs, Bug.o )
 ghc-stage2: panic! (the 'impossible' happened)
   (GHC version 8.5.20180511 for x86_64-unknown-linux):
         tcTyVarDetails
   co_aWx :: (k_aWt[tau:2] :: *) ~# (* :: *)
   Call stack:
       CallStack (from HasCallStack):
         callStackDoc, called at compiler/utils/Outputable.hs:1162:37 in
 ghc:Outputable
         pprPanic, called at compiler/basicTypes/Var.hs:497:22 in ghc:Var
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15142>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler