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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/113852] -Wsign-compare doesn't warn on unsigned result
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Date: Fri, 09 Feb 2024 16:40:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113852

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #2)
> I assume what's happening is that GCC assumes integer promotion from
> uint16_t to int is value preserving and so we get two positive values, and
> therefore comparison with an unsigned value is fine - there are no negati=
ve
> values involved and so it doesn't matter that we're comparing int with
> unsigned long. But of course that's not true here. We have two positive i=
nts
> but their product overflows to produce a negative int. I guess we're also
> assuming no overflow happens, because that would be undefined and we assu=
me
> no UB.

Ah yes, if you add -fwrapv then you get the -Wsign-compare warning, because=
 now
a negative product can occur without undefined overflow.=