`Returning address of local variable` on a C++ templated function with proper return type

The following code rises the warning: warning C4172: returning address of local variable or temporary but it makes no sense to me as the return type is a constant reference of the same type returned by std::vector::operator[] and std::vector::at().

Do you have an explanation for it?

#include <iostream>
#include <vector>

template <class V>
const V &at(const std::vector<V> &p_vec, const std::size_t p_index, const V &p_default) {
    if (p_vec.size() <= p_index) {
        return p_default;
    }
    return p_vec[p_index]; // warning C4172: returning address of local variable or temporary
}

int main(){
    std::vector<bool> the_vector;

    bool val = at(the_vector, 2, false);

    if (val){
        std::cout << "TRUE" << std::endl;
    }else{
        std::cout << "FALSE" << std::endl;
    }

    return 1;
}

Sample project: https://godbolt.org/z/KW5qo5YnK

>Solution :

The compiler is right to warn about this, because std::vector<bool>::operator[] returns std::vector<bool>::reference, which is not a reference to a bool. It is an implementation defined proxy type. The proxy can be converted to bool and thats the temporary the compiler mentions.

std::vector<bool> is specified such that it encourages implementations to pack 8 bools into a single byte. Hence operator[] cannot return a reference to bool.

You can specialize the function template for V==bool, or simply return a value.