False positive Condition 'addcnt!=255' is always true (uint8_t addcnt is decremented).

[Mario De Weerd]

#include <stdint.h>
#include <stdio.h>
#include <string.h>
char test[50];
static void test_255match()
    {
    uint8_t addcnt;
    for (addcnt = 0; addcnt < 10; addcnt++)
        {
        printf("%u\n", addcnt);
        if (test[addcnt] == '+')
            {
            test[addcnt] = 0;
            addcnt--;
            printf("AFTER: %u\n", addcnt);
            if (addcnt != 255)
                {
                printf("CONDITION MATCHED:%u\n", addcnt);
                }
            else
                {
                printf("CONDITION NOT MATCHED:%u\n", addcnt);
                }
            }
        }
    }

void main()
    {
    snprintf(test, sizeof(test), "A+CD");
    test_255match();
    snprintf(test, sizeof(test), "+BCD");
    test_255match();
    }

/*
 * With this testcase file called 'und.c':
 *
 * cppcheck --enable=style,portability,warning,performance,unusedFunction und.c
 * yields:
 * und.c:16:24: style: Condition 'addcnt!=255' is always true [knownConditionTrueFalse]
 *             if (addcnt != 255)
 *                        ^
 * und.c:14:13: note: addcnt is decremented', new value is 254
 *             addcnt--;
 *             ^
 * und.c:16:24: note: Condition 'addcnt!=255' is always true
 *             if (addcnt != 255)
 *                        ^
 *
 *
 * Demonstration that this is false positive:
 *
 * `gcc und.c ; ./a.exe` yields message "CONDITION MATCHED":
 *
 * 0
 * 1
 * AFTER: 0
 * CONDITION MATCHED:0
 * 1
 * 2
 * 3
 * 4
 * 5
 * 6
 * 7
 * 8
 * 9
 * 0
 * AFTER: 255
 * CONDITION NOT MATCHED:255
 * 0
 * 1
 * 2
 * 3
 * 4
 * 5
 * 6
 * 7
 * 8
 * 9
 */

[CHR]

Probably an instance of https://trac.cppcheck.net/ticket/10616