Describe the bug
When given the least possible signed integer value, gcd cannot make it positive, and returns a negative result.
According to [numeric.ops.gcd]/2, it is UB:

Preconditions: |m| and |n| are representable as a value of common_­type_­t<M, N>.
[Note 1: These requirements ensure, for example, that gcd(m, m)=|m| is representable as a value of type M. — end note]

According to [expr.const]/6 whereas language UB shouldn't compile, it is implementation-specific if library precondition violation compiles in constexpr context or not.

Since we are permitted do a favor and reject overflowing value, we should.

The same is for lcm, except that it should also check the result.

Command-line test case

#include <limits>
#include <numeric>
using namespace std;

auto test() {
    constexpr int x = numeric_limits<int>::lowest();
    constexpr int y = numeric_limits<int>::lowest();
    constexpr auto z = std::gcd(x, y);
    return z;
}

returns -1

Expected behavior
Don't compile

STL version

Version 17.0.0 Preview 6.0

Additional context
From this conversation https://discord.com/channels/737189251069771789/740318951766098012/902077925506039808