Sometimes it helps to test. Which I just did. :-)

Actually the compiler (at least clang) warns about this:

    $ gcc -W -Wall test.c -o test
    test.c:8:7: warning: multiple unsequenced modifications to 'a' [-Wunsequenced]
            a = a++ + ++a;
                 ^    ~~
    1 warning generated.

The undefined behaviour stems from the fact that "a" is modified multiple times between the "sequence points" (so it's irrelevant to the actual problem that this happens with ++, --, pre-, or, post-, or in which order) We only can modify the variable safely once on the right side without entering bizarro world.

A construct like this certainly can be confusing.