Alexandre, which Python version did you compile with -Wstrict-overflow?
 It would behoove us to check 2.5.2 thoroughly before it goes out the door.

I will contact Coverity to ask if they check for this kind of thing. 
(They just upgraded us to "Rung 2", whatever that may mean. :-)

MvL: I don't want 2s complement throughout the language, I just want the
overflow checks to be reliable.  Since I'd forgotten about the
difference between unsigned and signed overflow, I have no idea how many
overflow checks have been submitted that are relying on signed overflow;
though apparently (if the -Wstrict-overflow results can be trusted)
we're okay.

FWIW, I've heard that some commercial compilers (e.g. XLC) assume that
even *unsigned* overflow is undefined, violating the C standard.  This
would suggest that buffer overflow checks should be coded without
relying on arithmetic overflow at all.  This is possible, just a bit hairy.