This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 116a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-12-19

1353. Array and variant members and deleted special member functions

[Accepted as a DR at the June, 2023 meeting.]

The specification of when a defaulted special member function is to be defined as deleted sometimes overlooks variant and array members.

Proposed resolution (approved by CWG 2023-02-07):

1. Change in 11.4.5.2 [class.default.ctor] paragraph 2 as follows:

   A defaulted default constructor for class X is defined as deleted if:

   • X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
   • X is a non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
   • any non-static data member with no default member initializer (11.4 [class.mem]) is of reference type,
   • any non-variant non-static data member of const-qualified type (or possibly multi-dimensional array thereof) with no brace-or-equal-initializer is not const-default-constructible (9.4 [dcl.init]),
   • X is a union and all of its variant members are of const-qualified type (or possibly multi-dimensional array thereof),
   • X is a non-union class and all members of any anonymous union member are of const-qualified type (or possibly multi-dimensional array thereof),
   • any potentially constructed subobject, except for a non-static data member with a brace-or-equal-initializer or a variant member of a union where another non-static data member has a brace-or-equal-initializer, has class type M (or possibly multi-dimensional array thereof) and either M has no default constructor or overload resolution (12.2 [over.match]) as applied to find M's corresponding constructor results in an ambiguity or in a function that is deleted or inaccessible from the defaulted default constructor either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function, or
   • any potentially constructed subobject has a type with class type M (or possibly multi-dimensional array thereof) and M has a destructor that is deleted or inaccessible from the defaulted default constructor.

2. Change in 11.4.5.3 [class.copy.ctor] paragraph 10 as follows:

   ... A defaulted copy/move constructor for a class X is defined as deleted (9.5.3 [dcl.fct.def.delete]) if X has:

   • a potentially constructed subobject of type M (or possibly multi-dimensional array thereof) that cannot be copied/moved because for which overload resolution (12.2 [over.match]), as applied to find M's corresponding constructor, results in an ambiguity or a function that is deleted or inaccessible from the defaulted constructor either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function,
   • a variant member whose corresponding constructor as selected by overload resolution is non-trivial,
   • any potentially constructed subobject of a type with class type M (or possibly multi-dimensional array thereof) where M has a destructor that is deleted or inaccessible from the defaulted constructor, or,
   • for the copy constructor, a non-static data member of rvalue reference type.

3. Change in 11.4.6 [class.copy.assign] paragraph 7 as follows:

   A defaulted copy/move assignment operator for class X is defined as deleted if X has:

   • a variant member with a non-trivial corresponding assignment operator and X is a union-like class, or
   • a non-static data member of const non-class type (or possibly multi-dimensional array thereof), or
   • a non-static data member of reference type, or
   • a direct non-static data member of class type M (or possibly multi-dimensional array thereof) or a direct base class M that cannot be copied/moved because overload resolution (12.2 [over.match]), as applied to find M's corresponding assignment operator, results in an ambiguity or a function that is deleted or inaccessible from the defaulted assignment operator either does not result in a usable candidate (12.2.1 [over.match.general]) or, in the case of a variant member, selects a non-trivial function.

4. Change in 11.4.7 [class.dtor] paragraph 7 as follows:

   A defaulted destructor for a class X is defined as deleted if:

   • X is a union-like class that has a variant member with a non-trivial destructor,
   • any potentially constructed subobject has class type M (or possibly multi-dimensional array thereof) and M has a deleted destructor that is deleted or a destructor that is inaccessible from the defaulted destructor or, in the case of a variant member, is non-trivial,
   • or, for a virtual destructor, lookup of the non-array deallocation function results in an ambiguity or in a function that is deleted or inaccessible from the defaulted destructor.