lerp.cpp

// from this

template <typename T, typename U>
inline T Lerp(const T& a, const T& b, const U& t)
{
    return a * (1.0 - t) + b * t;
}

// to this:

template <typename T>
inline constexpr std::enable_if_t<std::is_floating_point_v<T>, T> Lerp(T a, T b, T t)
{
    if (isnan(a) || isnan(b) || isnan(t))
    {
        return std::numeric_limits<T>::quiet_NaN();
    }
    else if ((a <= T{0} && b >= T{0}) || (a >= T{0} && b <= T{0}))
    {
        // ab <= 0 but product could overflow.
        // return a * (T(1) - t) + b * t;
        return t * b + (T{1} - t) * a;
    }
    else if (t == T{1})
    {
        return b;
    }
    else
    {
        // monotonic near t == 1.
        const auto x = a + t * (b - a);

        return (t > T{1}) == (b > a) ? Max(b, x) : Min(b, x);
    }
}