Unsigned char automatically promoted to int in function call, why?

Question

Why is an unsigned char automatically promoted to an int when calling a function? In the example below there is an f(int) and a f(char) function. It seemed more logical that the compiler would coerce an unsigned char argument to a char and call f(char) since they have the same number of bits. It is calling f(int) instead, even when that means promoting the argument to a type with more bits. Any pointers to where the rule is defined? Standard or compiler/platform specific?

#include <iostream>

void f( int key )
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

void f( char key )
{
    std::cout << __PRETTY_FUNCTION__ << std::endl;
}

int main( int argc, char* argv[] )
{
    int a = 'a';
    char b = 'b';
    unsigned char c = 'c';

    f(a);
    f(b);
    f(c);

    return 0;
}

Produces this output:

void f(int)
void f(char)
void f(int)