C++ auto on int16_t casts to integer

The problem isn't with auto here. When you subtract two int16_t values, the result is an int. We can demonstrate it with this code here:

#include <iostream>
#include <cstdint>
using namespace std;

template<class T>
void print_type(T) {
    std::cout << __PRETTY_FUNCTION__ << std::endl; 
}

int main() {
    int16_t a = 10;
    int16_t b = 20;
    print_type(a);
    print_type(b);
    print_type(a - b); 
    return 0;
}

a and b are both short ints, but when you add or subtract them it produces a regular int. This is to help prevent overflow / and is also for backwards compatibility.

This phenomenon is called the usual arithmetic conversions. It is defined in the C and C++ standards and (roughly said) converts anything smaller than an int to an int. It converts larger types as well. Take some time and read about it, you'll need it quite often.