[GHC] #12933: Wrong class instance selection with Data.Kind.Type

6 Dec 2016

      #12933: Wrong class instance selection with Data.Kind.Type
-------------------------------------+-------------------------------------
           Reporter:  julm           |             Owner:
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:  TypeInType     |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If you consider the following code:
 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE FlexibleInstances #-}
 module Bug where

 import GHC.Exts (Constraint)
 import Data.Kind

 -- | Partial singleton for a kind type.
 data SKind k where
   SKiTy :: SKind Type
   SKiCo :: SKind Constraint

 instance Show (SKind k) where
   show SKiTy = "*"
   show SKiCo = "Constraint"

 class IKind k where
   kind :: SKind k
 instance IKind Constraint where
   kind = SKiCo
 }}}

 Then, the main below will compile even though there is no (IKind Type)
 instance, and it will print "Constraint" two times,
 instead of an expected "Constraint" then "*":
 {{{#!hs
 main :: IO ()
 main = do
   print (kind::SKind Constraint)
   print (kind::SKind Type)
 }}}

 And, the main below will print "*" two times,
 instead of an expected "*" then "Constraint":
 {{{#!hs
 instance IKind Type where
   kind = SKiTy

 main :: IO ()
 main = do
   print (kind::SKind Type)
   print (kind::SKind Constraint)
 }}}

 This can be worked around by replacing Type with a new data type Ty to
 select the right class instances, using two type families Ty_of_Type and
 Type_of_Ty, as done in the attached Workaround.hs.

 Sorry if this bug has already been fixed in HEAD: I was unable to find
 neither a bug report similar, nor a Linux x86_64 build of HEAD for me to
 test.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12933
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler