[GHC] #16313: Core Lint warning (Unsafe coercion: {left, right}-hand type is levity-polymorphic)

12 Feb 2019


      #16313: Core Lint warning (Unsafe coercion: {left,right}-hand type is levity-
polymorphic)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 goldfire claims [https://github.com/goldfirere/singletons/issues/383 this]
 is a GHC bug, so I'm reporting it here:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 module Bug where

 import Data.Kind (Type)
 import GHC.Exts (TYPE)
 import Type.Reflection (TypeRep, (:~~:)(..), eqTypeRep)
 import Unsafe.Coerce (unsafeCoerce)

 data SBool :: Bool -> Type where
   SFalse :: SBool False
   STrue  :: SBool True

 type family DefaultEq (a :: k) (b :: k) :: Bool where
   DefaultEq a a = 'True
   DefaultEq a b = 'False

 sEqTypeRep :: forall rep (x :: TYPE rep) (y :: TYPE rep).
               TypeRep x -> TypeRep y -> SBool (DefaultEq x y)
 sEqTypeRep tra trb =
   case eqTypeRep tra trb of
     Just HRefl -> STrue
     Nothing    -> unsafeCoerce SFalse
 }}}
 {{{
 $ /opt/ghc/8.6.3/bin/ghc Bug.hs -O -dcore-lint -fforce-recomp
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Float inwards ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Called arity analysis ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Demand analysis ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Worker Wrapper binds ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Exitification transformation ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Float out(FOS {Lam = Just 0,
                                                      Consts = True,
                                                      OverSatApps = True})
 ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Common sub-expression ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Float inwards ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Simplifier ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Demand analysis ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of Tidy Core ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 *** Core Lint warnings : in result of CorePrep ***
 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: left-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg

 <no location info>: warning:
     In a case alternative: (True)
     Unsafe coercion: right-hand type is levity-polymorphic
       From: x_a1rf
         To: y_a1rg
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16313
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler