[committed] libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978]

[Jonathan Wakely]

On 29/08/20 00:20 +0200, Daniel Krügler wrote:
>Am Sa., 29. Aug. 2020 um 00:12 Uhr schrieb Jonathan Wakely via
>Libstdc++ <libstdc++@gcc.gnu.org>:
>>
>> This fixes a bug with mixed signed and unsigned types, where converting
>> a negative value to the unsigned result type alters the value. The
>> solution is to obtain the absolute values of the arguments immediately
>> and to perform the actual GCD or LCM algorithm on two arguments of the
>> same type.
>>
>> In order to operate on the most negative number without overflow when
>> taking its absolute, use an unsigned type for the result of the abs
>> operation. For example, -INT_MIN will overflow, but -(unsigned)INT_MIN
>> is (unsigned)INT_MAX+1U which is the correct value.
>>
>> libstdc++-v3/ChangeLog:
>>
>>         PR libstdc++/92978
>>         * include/std/numeric (__abs_integral): Replace with ...
>>         (__detail::__absu): New function template that returns an
>>         unsigned type, guaranteeing it can represent the most
>>         negative signed value.
>>         (__detail::__gcd, __detail::__lcm): Require arguments to
>>         be unsigned and therefore already non-negative.
>>         (gcd, lcm): Convert arguments to absolute value as unsigned
>>         type before calling __detail::__gcd or __detail::__lcm.
>>         * include/experimental/numeric (gcd, lcm): Likewise.
>>         * testsuite/26_numerics/gcd/gcd_neg.cc: Adjust expected
>>         errors.
>>         * testsuite/26_numerics/lcm/lcm_neg.cc: Likewise.
>>         * testsuite/26_numerics/gcd/92978.cc: New test.
>>         * testsuite/26_numerics/lcm/92978.cc: New test.
>>         * testsuite/experimental/numeric/92978.cc: New test.
>>
>> Tested powerpc64le-linux. Committed to trunk.
>
>Shouldn't the overload of __absu
>
>void __absu(bool) = delete;
>
>still also be a template or is just the diff presentation confusing me?

Good point! It's called as __absu<U>(v) so it needs to be a function
template for the deleted one to be a candidate.

I'm not sure we really need it, since all the callers have a
static_assert ensuring it's not a bool. But if we have it, it should
be correct.