Re: [GHC] #12503: Template Haskell regression: GHC erroneously thinks a type variable is also a kind

10 Sep 2016

      #12503: Template Haskell regression: GHC erroneously thinks a type variable is also
a kind
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 It should be noted that you don't need a type/kind variable explicitly
 named `k` to trigger this bug. You can also trigger it with an arbitrarily
 named variable like so:

 {{{#!hs
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Regression2 where

 import Language.Haskell.TH

 data family T (a :: b)
 data instance T b
 class C a

 $(do FamilyI
 #if __GLASGOW_HASKELL__ >= 800
        (DataFamilyD tName _ _)
 #else
        (FamilyD _ tName _ _)
 #endif
        [DataInstD [] _ [tyVar]
 #if __GLASGOW_HASKELL__ >= 800
           _
 #endif
           _ _] <- reify ''T
      d <- instanceD (cxt []) (conT ''C `appT` (conT tName `appT` return
 tyVar)) []
      return [d])
 }}}

 {{{
 $ /opt/ghc/8.0.1/bin/ghc Regression2.hs
 [1 of 1] Compiling Regression2      ( Regression2.hs, Regression2.o )
 Regression2.hs:(15,3)-(27,15): Splicing declarations
     do { FamilyI (DataFamilyD tName_a2RY _ _)
                  [DataInstD [] _ [tyVar_a2RZ] _ _ _] <- reify ''T;
          d_a322 <- instanceD
                      (cxt [])
                      (conT ''C `appT` (conT tName_a2RY `appT` return
 tyVar_a2RZ))
                      [];
          return [d_a322] }
   ======>
     instance C (T (b_avD :: b_avO))

 Regression2.hs:15:3: error:
     Variable ‘b_avD’ used as both a kind and a type
     Did you intend to use TypeInType?
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12503#comment:2
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