Re: [GHC] #14904: Compiler panic (piResultTy)

3 May 2018

      #14904: Compiler panic (piResultTy)
-------------------------------------+-------------------------------------
        Reporter:  kcsongor          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.2
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14873            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * related:  14873 => #14873

Comment:

 It looks like commit faec8d358985e5d0bf363bd96f23fe76c9e281f7 (`Track type
 variable scope more carefully.`) nabbed this one. After that commit, I get
 error messages instead of panics on each program in the ticket.

 For the first program:

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

 import Data.Kind

 type family F (f :: forall a. g a) :: Type where
   F (f :: forall a. g a) = Int
 }}}
 {{{
 $ ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:8:1: error:
     You have written a *complete user-suppled kind signature*,
     but the following variable is undetermined: k0 :: *
     Perhaps add a kind signature.
     Inferred kinds of user-written variables:
       g :: k0 -> *
       f :: forall (a :: k0). g a
   |
 8 | type family F (f :: forall a. g a) :: Type where
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 For the second program:

 {{{#!hs
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeInType #-}
 module Bug2 where

 import Data.Kind

 type family F f :: Type where
   F ((f :: forall a. g a) :: forall a. g a) = Int
 }}}
 {{{
 $ ghc/inplace/bin/ghc-stage2 Bug2.hs
 [1 of 1] Compiling Bug2             ( Bug2.hs, Bug2.o )

 Bug2.hs:9:7: error:
     • Expected kind ‘forall (a :: k1). g a’, but ‘f’ has kind ‘k0’
     • In the first argument of ‘F’, namely
         ‘((f :: forall a. g a) :: forall a. g a)’
       In the type family declaration for ‘F’
   |
 9 |   F ((f :: forall a. g a) :: forall a. g a) = Int
   |       ^
 }}}

 I'll add regression tests and close this.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14904#comment:2
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