In practice, I cannot see any reason why it would be explicitly disallowed (I haven’t used -O2 as far back as I can remember); it also discourages doing silly and minor optimizations in code.
I would say (after some googling), that it is because of historical reasons and maybe by intention to make life of C/C++ coders easier - as stated in one of the links:
It (
-O3) does however tend to reveal cases where people rely on undefined behavior, due to relying more strictly on the rules, and especially corner cases, of the language(s).
Can you give a more concrete example of what exactly you are trying to do ?
Maybe some one have a clue here, there are clever people around 
While waiting, you can ask CodeChef team via e-mail (feedback@codechef.com) and let as know when they reply (it may take some time).
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That is why I brought up having the option to do it vs. forced to do it. I understand that in some hypothetical cases it can cause errors, but from my experience this is pretty rare.
Here’s a concrete example for C99 since it may have been unclear:
gcc -Werror -pedantic-errors -std=c99 -O3 -fomit-frame-pointer -o prog prog.c