Overload unary `&` only as part of a system of smart pointers and references

Reason

The & operator is fundamental in C++. Many parts of the C++ semantics assume its default meaning.

Example

class Ptr { // a somewhat smart pointer
    Ptr(X* pp) : p(pp) { /* check */ }
    X* operator->() { /* check */ return p; }
    X operator[](int i);
    X operator*();
private:
    T* p;
};

class X {
    Ptr operator&() { return Ptr{this}; }
    // ...
};

Note

If you "mess with" operator & be sure that its definition has matching meanings for ->, [], *, and . on the result type. Note that operator . currently cannot be overloaded so a perfect system is impossible. We hope to remedy that: Operator Dot (R2). Note that std::addressof() always yields a built-in pointer.

Enforcement

Tricky. Warn if & is user-defined without also defining -> for the result type.