The surprising semantics of integral promotion

gistfile1.txt

#include <stdio.h>

int
main()
{
	unsigned char a = 1, b = 2, c = 0;

	if (a - b >= c)
		printf("Expression true without explicit cast.\n");

	if ((unsigned char)(a - b) >= c)
		printf("Expression true with explicit cast.\n");

	return (0);
}