[GHC] #8037: GHC panic when compiling unsafeCoerce

[GHC]

#8037: GHC panic when compiling unsafeCoerce
---------------------------+------------------------------------------------
Reporter:  ghc@…           |          Owner:                  
    Type:  bug             |         Status:  new             
Priority:  normal          |      Component:  Compiler        
 Version:  7.6.1           |       Keywords:                  
      Os:  MacOS X         |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown    |      Blockedby:                  
Blocking:                  |        Related:                  
---------------------------+------------------------------------------------
 The GHC compiler crashes on the following program.

 {{{
 module Test where

 import Unsafe.Coerce
 import Graphics.Rendering.OpenGL (color, Color3, GLdouble)

 data D4 = D4 Double Double Double Double

 crash :: D4 -> IO ()
 crash c = color (invalidCast c)

 invalidCast :: D4 -> Color3 GLdouble
 invalidCast = unsafeCoerce
 }}}

 With the following output:

 {{{
 [1 of 1] Compiling Test             ( Test.hs, Test.o )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.6.1 for x86_64-apple-darwin):
         cgLookupPanic (probably invalid Core; try -dcore-lint)
     ww_s28U{v} [lid]
     static binds for:
     local binds for:
 }}}

 I've made several attempts to make the input program smaller, but all
 modifications will result in successful compilation.

 The cast is invalid but should produce an runtime error, not a compile
 time error.

 Changes I made that somehow prevent this bug from appearing:

   - Changing the `crash` functions into point free style `color .
 invalidCast`.
   - Decreasing the amount of doubles in the `D4` datatype.
   - Inlining the exact definitions from the OpenGL package.
   - Using the `print` function instead of the `color` function.

 (btw, I know I shouldn't use unsafeCoerce)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/8037>
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