I found when I using a concept with requires expression, the compiler won’t match the constraints but report an ambiguous error. After replace the requires expression to a common expression, it actually passed the compile. What is the difference between adding a requires expression to a concept and not adding one ?

Here is the demo code:

#include <cstdint>

class uint63;

template<typename T>
/*
// this got error
concept is_not_uint63_t = requires {
    !std::same_as<T, uint63>;
};
*/
// and this pass the compile
concept is_not_uint63_t = !std::same_as<T, uint63>;


class uint63 {
    template<is_not_uint63_t T>
    friend uint63 operator +(T t, uint63 u) {
        return u.container_ + static_cast<uint64_t>(t);
    }

private:
    uint64_t container_;
public:
    uint63(uint64_t u) : container_(u) {}
    operator uint64_t () {
        return this->container_;
    }
    template<typename T> uint63 operator +(T n) const { return { this->container_ + static_cast<uint64_t>(n) }; }
};
int main() {
    uint63 k = 1;
    uint63 w = k + 1;
    uint63 r = k + w;
}