Here is my code:

#include<iostream>

template <typename T1, typename T2>
auto sum(const T1 &v1, const T2 &v2) -> decltype(v1 + v2)
{
    return v1 + v2;
}

int main()
{
    std::cout << sum(12, 123456789123456789123456789123456789123456789) << std::endl;
    return 0;
}

Here is the compiler's complaint:

[Error] ambiguous overload for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream}' and '')

When I changed the seconed argument of sum to 12, this code compiled without complaint. I nee