[GHC] #2166: minBound `rem` (-1) fails

[GHC]

#2166: minBound `rem` (-1) fails
--------------------------+-------------------------------------------------
    Reporter:  atsampson  |       Owner:                
        Type:  bug        |      Status:  new           
    Priority:  normal     |   Component:  libraries/base
     Version:  6.8.2      |    Severity:  minor         
    Keywords:             |    Testcase:                
Architecture:  x86        |          Os:  Linux         
--------------------------+-------------------------------------------------
 I would expect both of the following operations to return 0, on the
 grounds that anything rem/mod -1 is zero:

 {{{
 Prelude> (minBound :: Int) `rem` (-1)
 *** Exception: arithmetic overflow
 Prelude> (minBound :: Int) `mod` (-1)
 *** Exception: arithmetic overflow
 }}}

 In GHC 6.8.2, the instances of Integral for the various integer types
 explicitly check for this case and throw the error:

 {{{
     rem     x@(I32# x#) y@(I32# y#)
         | y == 0                  = divZeroError
         | x == minBound && y == (-1) = overflowError
         | otherwise               = I32# (x# `remInt32#` y#)
 }}}

 The current behaviour makes some sense in that rem/mod's behaviours are
 defined in terms of quot/div which can't give a sensible output for these
 input values -- but I can't really see anything wrong with changing the
 second guard there to return {{{0}}}.

 (I ran into this while implementing constant expression folding in a
 compiler for the occam programming language; the occam compiler test suite
 explicitly checks that {{{minBound `rem` (-1) == 0}}}.)

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2166>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler