Integer promotion between unsigned and int

Here

std::string s{ "aa" }; s.length - 3;  // == very large number

From memory, C's integer promotion rules (idk about C++) produce a type wide enough to fit the result of the calculation and favour unsigned types (for the extra bit of width). But 3 is of type int (right?). So why is the result of type unsigned instead of int or long? unsigned is certainly not wide enough to capture the result of the expression!