[GHC] #12222: ghc panic kindFunResult with template haskell 'isInstance'

22 Jun 2016

      #12222: ghc panic kindFunResult with template haskell 'isInstance'
-------------------------------------+-------------------------------------
           Reporter:  ghorn          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  7.10.3
  Haskell                            |
           Keywords:  kindFunResult  |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Compile-time
  Unknown/Multiple                   |  crash
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 --TH.hs

 {-# OPTIONS_GHC -Wall #-}
 {-# LANGUAGE TemplateHaskell #-}

 module TH
        ( SomeClass
        , doThStuff
        ) where

 import Control.Monad ( void )
 import Language.Haskell.TH

 class SomeClass a where

 doThStuff :: Name -> Q [Dec]
 doThStuff name = reify name >>= go

 go :: Info -> Q [Dec]
 go (TyConI (DataD [] _ _ [(NormalC _ [(_,typ)])] _)) = do
   void (isInstance ''SomeClass [typ])  -- THIS LINE CRASHES GHC
   return []
 go _ = fail "wrong info"
 }}}

 {{{#!hs
 -- Bug.hs

 {-# OPTIONS_GHC -Wall #-}
 {-# LANGUAGE TemplateHaskell #-}

 module Bug
        ( Bar(..)
        ) where

 import TH

 data Foo a = Foo a
 instance SomeClass (Foo a)

 data Bar f = Bar (Foo (f Int))
 doThStuff ''Bar
 }}}

 I get this error:
 {{{
 $ runghc -ddump-splices Bug.hs

 Bug.hs:1:1:
     Exception when trying to run compile-time code:
       ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         kindFunResult

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

     Code: doThStuff ''Bar
 }}}

 If I comment out either `instance SomeClass (Foo a)` or `void (isInstance
 ''SomeClass [typ])`, the crash does not occur.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12222
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler