This is UB and it's caught by both GCC and Clang:

    $ gcc -Wall test.c 
    test.c: In function ‘main’:
    test.c:12:13: warning: operation on ‘a’ may be undefined [-Wsequence-point]
      test(a++, a++);
                ~^~
    $ clang test.c
    test.c:12:8: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
            test(a++, a++);
                  ^    ~~
    1 warning generated.