https://godbolt.org/g/nfsn8j

assignment_operators.cpp

// https://www.youtube.com/watch?v=2gjroKLyWKE

#include <utility>

struct S
{
    // trailing & means only applies to lvalues on left side
    S& operator=(const S& s) & = default;
    S& operator=(S& s) & = default;
    S& operator=(S&& s) & = default; // move from
    S& operator=(const S&& s) & = delete; // can't move from const rvalue, no default

    // trailing && means only applies to rvalues on left side
    S& operator=(const S& s) && = delete; // nonsensical to assign to temp (int can't)
    S& operator=(S& s) && = delete; // nonsensical to assign to temp (int can't)
    S& operator=(S&& s) && = delete; // (i) nonsensical to assign to temp (int can't)
    S& operator=(const S&& s) && = delete; // can't move from const rvalue, no default

    // const versions, but assigning to const makes no sense, no defaults
    // trailing & means only applies to lvalues on left side
    S& operator=(const S& s) const & = delete;
    S& operator=(S& s)  const & = delete;
    S& operator=(S&& s) const & = delete;
    S& operator=(const S&& s) const & = delete; // can't move from const

    // const versions, but assigning to const makes no sense, no defaults
    // trailing && means only applies to rvalues on left side
    S& operator=(const S& s) const && = delete;
    S& operator=(S& s) const && = delete;
    S& operator=(S&& s) const && = delete;
    S& operator=(const S&& s) const && = delete; // can't move from const
};

int main()
{
    //S{} = S{}; // works if (i) is defaulted
    S s{};
    S s2{};
    s = std::move(s2);
    return 0;
}