Draft of optional{float} for safe nan usage

float_opt.cpp

#include <cmath>
#include <limits>

struct float_opt
{
    float value_ = std::numeric_limits<float>::quiet_NaN();
    
    explicit float_opt(float v): value_(v) {}
    
    constexpr float_opt() = default;
    
    constexpr float unwrap() const { return value_; }
    
    bool valid() const { return !std::isnan(value_); }
    bool empty() const { return std::isnan(value_); }
};

inline bool operator==(float_opt lhs, float_opt rhs) {
    return lhs.unwrap() == rhs.unwrap() || (lhs.empty() && rhs.empty());
}

inline bool operator!=(float_opt lhs, float_opt rhs) {
    return !(lhs == rhs);
}

inline bool operator==(float_opt lhs, float rhs) {
    return lhs == float_opt{rhs};
}

inline bool operator!=(float_opt lhs, float rhs) {
    return !(lhs == rhs);
}

inline bool operator==(float lhs, float_opt rhs) {
    return float_opt{lhs} == rhs;
}

inline bool operator!=(float lhs, float_opt rhs) {
    return !(lhs == rhs);
}