-Wconversion producing incorrect warning

[Kevin P. Fleming]

When compiling this program:

#include <sys/types.h>
#include <inttypes.h>

typedef uint16_t t;

int f(t x)
{
	return x*x;
}

int main(int argc, char *argv[])
{
	t b = 12;

	return f(b);
}

using GCC 3.3.5, 3.4.3 and 4.0.1 with '-Wconversion' in effect, I get 
this warning:

luigi.c: In function `main':
luigi.c:15: warning: passing arg 1 of `f' with different width due to 
prototype

I don't understand why this is true... the prototype and the argument 
are of the same type.