I dont know either.

int somearray[10];

new_ptr = somearray + signed_value;

or

element = somearray[signedvalue];

this seems almost criminal to how my brain does logic/C code.

The only thing i could think of is this:

somearray+=11; somearray[-1] // index set to somearray[10] ??

if i'd see my CPU execute that i'd want it to please stop. I'd want my compiler to shout at me like a little child, and be mean until i do better.

-Wall -Wextra -Wextra -Wpedantic <-- that should flag i think any of these weird practices.

As you stated tho, i'd be keen to learn why i am wrong!