Segmentation Fault caused by redefined operator '='

Question

I compiled this program by g++ and clang++.
G++ executable print one "trigger A copy assignment".
Clang++ executable print two "trigger A copy assignment" and has a segmentation fault.
Does there has any ill-formed? Or is it only clang's problem?

version: gcc-7.4.0/clang-10.0.0

#include <iostream>
struct A {
  A &operator =(const A &other) {
    std::cout << "trigger A copy assignment\n";
  }
};

int main() {
  A x,y;
  y = x;
}

Answer 1

While not formally ill-formed, your code does create undefined behaviour , as your assignment operator function, declared as returning an A& , doesn't actually return anything!

From cppreference :

Flowing off the end of a value-returning function (except main) without a return statement is undefined behavior.

This minor code amendment will prevent the clang crash (but maybe won't do what you want):

struct A {
    A& operator =(const A& other) {
        std::cout << "trigger A copy assignment\n";
        return *this; // MUST return SOMETHING!
    }
};