https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92978
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-9 branch has been updated by Jonathan Wakely <r...@gcc.gnu.org>: https://gcc.gnu.org/g:b3043e490896ea37cd0273e6e149c3eeb3298720 commit r9-8928-gb3043e490896ea37cd0273e6e149c3eeb3298720 Author: Jonathan Wakely <jwak...@redhat.com> Date: Fri Aug 28 22:45:24 2020 +0100 libstdc++: Fix std::gcd and std::lcm for unsigned integers [PR 92978] 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. (cherry picked from commit 82db1a42e9254c9009bbf8ac01366da4d1ab6df5)
