Fix #12992 (Refactoring: Use l-value reference specifier for assignment operators to prevent misuse)

[danmar]

Misra C++ 2023 15.0.2

[danmar]

@firewave @chrchr-github do you have any opinion about this? It prevents some misuse, compilers will not allow the code below:

struct C {
    C& operator=(const C& c) &;
    int x;
};

C getC();

int main() {
    getC() = getC();
    return 0;
}

[danmar]

For your information, I want that we use cppcheck premium for a self check. And activate Misra C++ 2023. However there are many Misra C++ rules that don't feel "natural" for Cppcheck source code to follow and it's 100% fine to suppress those rules.

[chrchr-github]

I think it's OK to do this.
Apparently, built-in types are not susceptible to this issue, but only user-defined ones.

int f();
struct S {};
S g();
int main() {
    f() = f(); // error
    g() = g(); // compiles
}