int main(){([](){}()));} Why does it run normally?

That code is called empty lambda. I don't really do anything.

First, [](){} is empty lambda expression. For convenience, let 1 = [](){} and (([](){})(); = ((A)();.

Second, parentheses expression in C/C++ have no effect on the code. It does the same thing, whether it's in parentheses or not. So you know up to here (A) = A. Let's say B = (A) and ((A)(); = (B());.

Third, () is (empty) lambda. Then let's say C = B() and (B(); = (C);.

(C) is parentheses the entire expression as we did in the second. Then D = (C) and (C); = D;.

Lastly. ; is the role to end a sentence.