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#include <iostream>
 
class Foo {
public:
    Foo() = default;
 
    Foo(const Foo&) = default;
    Foo(Foo&&) noexcept = default;
 
    Foo& operator=(const Foo& rhs) & {
        std::cout << "copy\n";
 
        return *this;
    }
 
    Foo& operator=(Foo&& rhs) & noexcept {
        std::cout << "move\n";
 
        return *this;
    }
 
    Foo operator+(const Foo& rhs) const {
        Foo x; // with some calculation
 
        return x;
    }
};
 
int main() {
    Foo a, b, c;
 
    a + b = c;
}

prog.cpp: In function ‘int main()’:
prog.cpp:32:13: error: passing ‘Foo’ as ‘this’ argument discards qualifiers [-fpermissive]
     a + b = c;
             ^
prog.cpp:10:10: note:   in call to ‘Foo& Foo::operator=(const Foo&) &’
     Foo& operator=(const Foo& rhs) & {
          ^~~~~~~~

Standard output is empty