https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92978
Bug ID: 92978
Summary: std::gcd mishandles mixed-signedness
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: rs2740 at gmail dot com
Target Milestone: ---
From https://stackoverflow.com/q/59379703/2756719; the current gcd
implementation is essentially
template<typename _Mn, typename _Nn>
constexpr common_type_t<_Mn, _Nn>
__gcd(_Mn __m, _Nn __n)
{
return __m == 0 ? __detail::__abs_integral(__n)
: __n == 0 ? __detail::__abs_integral(__m)
: __detail::__gcd(__n, __m % __n);
}
__m % __n generally does the wrong thing if one of the operands is negative and
signed and the other operand is unsigned. E.g., -120 % 10u is 6u, rather than
zero.
