I get errors when I use expressions in square brakets.

For example I have such operator:
p[tmp-1]=i;
Two times I got Crash (ACCESS_VIOLATION) and once I got TLE.

Today I change this operator to:
--tmp;p[tmp]=1;
and i get AC!!!
I didn't changed anything else! Variable tmp is no more used.

What's wrong with C++ compiler (or maybe with me)?
Which compiler is used?