Why does Release/Debug have a different result for std::min?

In IEEE 754 comparing NAN to anything will always yield false, no matter what it is.

slope > 0; // false
slope < 0; // false
slope == 0; // false

And, more importantly for you

slope < DBL_MAX; // false
DBL_MAX < slope; // false

So it seems that the compiler reorders the parameters/uses > or <= instead of <, and that’s why you get the differing results.

For example, those functions could be described as such

Release:

double const& min(double const& l, double const r) {
    return l <= r ? l : r;
}

Debug:

double const& min(double const& l, double const& r) {
    return r < l ? r : l;
}

The requirements (LessThanComparable) on std::min aside, those have the same meaning arithmetically. But they yield different results when you use them with NaN.