Default move constructor/assignment and deleted copy constructor/assignment

Question

According to the standard,

If the definition of a class X does not explicitly declare a move constructor, one will be implicitly declared as defaulted if

Answer 1

The behavior can essentially be illustrated with:

void foo(const int& i)
{
    std::cout << "copy";
}

void foo(int&& i)
{
    std::cout << "move";
}

When both overloads are present, the int&& overload is selected for rvalues while the const int& overload is selected for lvalues. If you remove the int&& overload, const int& is called (ergo, not an error) even for rvalues. This is essentially what's happening in this case:

class Foo
{
public:
  Foo() = default;
  Foo(Foo const &) = delete;
};

Overload resolution only sees one candidate but since it is explicitly deleted, the program is ill-formed.

The non-normative note below the section you quoted clarifies this is what's happening:

[ Note: When the move constructor is not implicitly declared or explicitly supplied, expressions that otherwise would have invoked the move constructor may instead invoke a copy constructor. — end note ]