Re: [GHC] #11334: GHC panic when calling typeOf on a promoted data constructor

27 Jan 2016

      #11334: GHC panic when calling typeOf on a promoted data constructor
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash                              |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1757
       Wiki Page:                    |
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Comment (by simonpj):

 Easier example is good (comment:11).

 Hint: always debug a compiler built with `-DDEBUG`.  My build shows
 {{{
 decideKindGeneralisationPlan
   type: (Proxy
            k1_aCf[tau:5]
            ('Other
               k_aCn[tau:5]
               f_aCo[tau:5]
               a_aCp[tau:5] |>  Other k_aCn[tau:5] f_aCo[tau:5]
 a_aCp[tau:5]>_N) |> <*>_N)
   ftvs: [k1_aCf[tau:5], f_aCo[tau:5], a_aCp[tau:5], k_aCn[tau:5]]
   should gen? True
 writeMetaTyVar k_aCn[tau:5] := *
 writeMetaTyVar f_aCo[tau:5] := *
 WARNING: file compiler\typecheck\TcMType.hs, line 553
   Ill-kinded update to meta tyvar
     f_aCo[tau:5] :: k_aCn[tau:5] -> *
                     * -> * := * :: *
                                    *
 writeMetaTyVar a_aCp[tau:5] := *
 quantifyTyVars
   globals: []
   nondep: []
   dep: [k_aCn[tau:5], f_aCo[tau:5], a_aCp[tau:5]]
   dep2: []
   quant_vars: []
 }}}
 Note the `WARNING`.

 The bug is this code in `quantifyTyVars`:
 {{{
              -- In the non-PolyKinds case, default the kind variables
              -- to *, and zonk the tyvars as usual.  Notice that this
              -- may make quantifyTyVars return a shorter list
              -- than it was passed, but that's ok
        ; poly_kinds <- xoptM LangExt.PolyKinds
        ; dep_vars2 <- if poly_kinds
                       then return dep_kvs
                       else do { let (meta_kvs, skolem_kvs) = partition
 is_meta dep_kvs
                                     is_meta kv = isTcTyVar kv &&
 isMetaTyVar kv

                               ; mapM_ defaultKindVar meta_kvs
                               ; return skolem_kvs }  -- should be empty
 }}}
 But in this case the variables we are quantifying over (I hardly know
 whether to call them type or kind variables now) are:
 {{{
 k_aCn :: *
 f_aCo :: k_aCn -> *
 a_aCp :: k_aCn
 }}}
 These appear in the user-written type signature which elaborates to:
 {{{
 Proxy ... (Other k_aCn f_aCo a_aCp)
 }}}
 It's clearly NOT RIGHT in the non-poly-kind case to default the "kind"
 variables to `*`.  I guess we have to use `Any` in the same way that we do
 for types.

 Can't do more tonight.  Richard how are you set?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11334#comment:12
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