Question

GCC (4.7.2) with -Wextra signals the following warning/error (I have -Werror enabled):

Comparison is always true due to limited range of data type [-Wtype-limits]

for the following code [try online]:

template <
    typename T,
    std::size_t N,
    bool = static_cast<std::size_t>(std::numeric_limits<T>::max()) < N>
struct validator {
    static constexpr bool validate(T value) {
        return static_cast<std::size_t>(value) < N;
    }
};

template <typename T, std::size_t N>
struct validator<T, N, true> {
    static constexpr bool validate(T) {
        return true;
    }
};

int main() {
    // Works
    static_assert(validator<int, 4>::validate(3), "Invalid");
    // Error :-(
    static_assert(validator<bool, 2>::validate(true), "Invalid");
}

I understand why the warning would occur in a normal expression context, e.g. when I would have used the following validate function:

template <typename T, std::size_t N>
bool validate(T value) {
    return static_cast<std::size_t>(value) < N;
}

– In fact, that’s why I am using the specialised template in the first place (and note that the correct template specialisation is used, and the error in my first code is raised by the template argument, not inside the function in the unspecialised template). Is there a way around this warning? If not, isn’t that a bug in the compiler?

Answer 1

This has been fixed in GCC trunk, see PR 11856

So wait for approximately late April and use GCC 4.8 🙂