Why is the move constructor neither declared nor deleted with clang?

Question

Consider the following classes.

struct with_copy {
    with_copy() = default;
    with_copy(with_copy const&) {}
    with_copy& operator=(with_copy c         


        

Answer 1

C++11, or rather n3485, [class.copy]/9:

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

• X does not have a user-declared copy constructor,
• X does not have a user-declared copy assignment operator,
• X does not have a user-declared move assignment operator,
• X does not have a user-declared destructor, and
• the move constructor would not be implicitly defined as deleted.

and /11:

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/ move constructor for a class X is defined as deleted (8.4.3) if X has:

• [...]
• for the copy constructor, a non-static data member of rvalue reference type, or
• for the move constructor, a non-static data member or direct or virtual base class with a type that does not have a move constructor and is not trivially copyable.

As with_copy is not trivially copyable, foo will have no move-constructor (it would be defined as deleted, therefore it won't be implicitly declared).

---

C++1y, or rather github repo commit e31867c0 from 2013-11-12; incorporating DR1402:

/9:

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

• X does not have a user-declared copy constructor,
• X does not have a user-declared copy assignment operator,
• X does not have a user-declared move assignment operator, and
• X does not have a user-declared destructor.

and /11:

An implicitly-declared copy/move constructor is an inline public member of its class. A defaulted copy/ move constructor for a class X is defined as deleted (8.4.3) if X has:

• [...]
• for the copy constructor, a non-static data member of rvalue reference type.

A defaulted move constructor that is defined as deleted is ignored by overload resolution (13.3, 13.4).

Here, foo will have a move-constructor.