Looks like a compiler bug in -O3 to me. Compare 165769461 and 165769473.

UPD: Not a compiler bug, see dedsec_29's response and my comment below.

It's not a compiler bug, if you run this with undefined sanitizer (-fsanitize=undefined) it shows a left shift on a negative value (on the first line of the combine function)

KAN has noticed that your Data(char c) constructor does not initialize fields. They're read from afterwards, though, which is UB. See 165775020

Hence, not a compiler bug. My recommendation is to always initialize all fields:

struct Data {
    long long ans = 0, cnto = 0, cntc = 0, anso = 0, ansc = 0;
 
    Data() {}
 
    Data(char c) {
        cnto = (c == '(');
        cntc = (c == ')');
    }
 
    Data(long long ans_,long long o_,long long c_,long long anso_,long long ansc_) {
        ans = ans_;
        cnto = o_;
        cntc = c_;
        anso = anso_;
        ansc = ansc_;
    }
};