This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of C++17 status.
Section: 20.15.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-10 Last modified: 2017-07-30
Priority: 2
View all other issues in [meta.logical].
View all issues with C++17 status.
Discussion:
The specification of conjunction and disjunction uses the term BaseCharacteristic, which is problematic in several ways:
That term is defined in 20.15.2 [meta.rqmts], but only for UnaryTypeTraits and BinaryTypeTraits. conjunction and disjunction seem to be neither.
20.15.2 [meta.rqmts] also requires the BaseCharacteristic for both UnaryTypeTraits and BinaryTypeTraits to be a specialization of integral_constant, which is inconsistent with the current design of conjunction and disjunction.
The requirement in 20.15.2 [meta.rqmts] that "member names of the BaseCharacteristic shall not be hidden and shall be unambiguously available" seems impossible to meet in every case, since the arbitrary base class from which a specialization of conjunction or disjunction derives may contain members called conjunction or disjunction that will necessarily be hidden.
[2016-08 Chicago]
Ville provided wording for both 2567 and 2568
Previous resolution [SUPERSEDED]:
In [meta.logical]/3, edit as follows:
The
BaseCharacteristic of aspecialization conjunction<B1, ..., BN> has a public and unambiguous base that is the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or if every Bi::value != false, the aforementioned baseBaseCharacteristicis the last type in the list. [ Note: This means a specialization of conjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]In [meta.logical]/6, edit as follows:
The
BaseCharacteristic of aspecialization disjunction<B1, ..., BN> has a public and unambiguous base that is the first type Bi in the list false_type, B1, ..., BN for which Bi::value != false, or if every Bi::value == false, the aforementioned baseBaseCharacteristicis the last type in the list. [ Note: This means a specialization of disjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]
Previous resolution [SUPERSEDED]:
Merged the resolution of 2587 with this issue. This proposed resolution resolves both, and includes fixes from Daniel for negation. Last review of this with LWG turned up a true_type typo in the definition of disjunction, and some editorial changes.In [meta.logical]/3, edit as follows:
The
BaseCharacteristic of aspecialization conjunction<B1, ..., BN> has a public and unambiguous base that is either
* the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or
* if there is no such Bi, the last type in the list.
is the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or if every Bi::value != false, the BaseCharacteristic is the last type in the list.
[ Note: This means a specialization of conjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]In [meta.logical]/6, edit as follows:
The
BaseCharacteristic of aspecialization disjunction<B1, ..., BN> has a public and unambiguous base that is either
* the first type Bi in the list true_type, B1, ..., BN for which Bi::value != false, or
* if there is no such Bi, the last type in the list.
is the first type Bi in the list true_type, B1, ..., BN for which Bi::value != false, or if every Bi::value == false, the BaseCharacteristic is the last type in the list.
[ Note: This means a specialization of disjunction does not necessarilyhave a BaseCharacteristic ofderive from either true_type or false_type. — end note ]
Previous resolution [SUPERSEDED]:
This wording is relative to N4606.
In 20.15.9 [meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };-3- The
BaseCharacteristic of aspecialization conjunction<B1, ..., BN> has a public and unambiguous base that is either
- the first type Bi in the list true_type, B1, ..., BN for which bool(Bi::value) is false, or
- if there is no such Bi, the last type in the list.
is the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or if every Bi::value != false, the BaseCharacteristic is the last type in the list.-?- The member names of the base class, other than conjunction and operator=, shall not be hidden and shall be unambiguously available in conjunction. [Note: This means a specialization of conjunction does not necessarily
have a BaseCharacteristic ofinherit from either true_type or false_type. —end note]In 20.15.9 [meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };-6- The
BaseCharacteristic of aspecialization disjunction<B1, ..., BN> has a public and unambiguous base that is either
- the first type Bi in the list true_type, B1, ..., BN for which bool(Bi::value) is true, or,
- if there is no such Bi, the last type in the list.
is the first type Bi in the list true_type, B1, ..., BN for which Bi::value != false, or if every Bi::value == false, the BaseCharacteristic is the last type in the list.-?- The member names of the base class, other than disjunction and operator=, shall not be hidden and shall be unambiguously available in disjunction. [Note: This means a specialization of disjunction does not necessarily
have a BaseCharacteristic ofinherit from either true_type or false_type. —end note]In 20.15.9 [meta.logical] p8, edit as follows
template<class B> struct negation : bool_constant<!bool(B::value)> { };-8- The class template negation forms the logical negation of its template type argument. The type negation<B> is a UnaryTypeTrait with a BaseCharacteristic of bool_constant<!bool(B::value)>.
[2016-08-03 Chicago]
Fri AM: Moved to Tentatively Ready
Proposed resolution:
This wording is relative to N4606.
In 20.15.9 [meta.logical] p3, edit as follows:
template<class... B> struct conjunction : see below { };
[…]
-3- The
BaseCharacteristic of aspecialization conjunction<B1, ..., BN> has a public and unambiguous base that is either
- — the first type Bi in the list true_type, B1, ..., BN for which bool(Bi::value) is false, or
- — if there is no such Bi, the last type in the list.
is the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or if every Bi::value != false, the BaseCharacteristic is the last type in the list.[Note: This means a specialization of conjunction does not necessarilyhave a BaseCharacteristic ofinherit from either true_type or false_type. —end note]-?- The member names of the base class, other than conjunction and operator=, shall not be hidden and shall be unambiguously available in conjunction.
In 20.15.9 [meta.logical] p6, edit as follows:
template<class... B> struct disjunction : see below { };
[…]
-6- The
BaseCharacteristic of aspecialization disjunction<B1, ..., BN> has a public and unambiguous base that is either
- — the first type Bi in the list false_type, B1, ..., BN for which bool(Bi::value) is true, or,
- — if there is no such Bi, the last type in the list.
is the first type Bi in the list false_type, B1, ..., BN for which Bi::value != false, or if every Bi::value == false, the BaseCharacteristic is the last type in the list.[Note: This means a specialization of disjunction does not necessarilyhave a BaseCharacteristic ofinherit from either true_type or false_type. —end note]-?- The member names of the base class, other than disjunction and operator=, shall not be hidden and shall be unambiguously available in disjunction.
In 20.15.9 [meta.logical] p8, edit as follows
template<class B> struct negation : see belowbool_constant<!B::value>{ };
-8- The class template negation forms the logical negation of its template type argument. The type negation<B> is a UnaryTypeTrait with a BaseCharacteristic of bool_constant<!bool(B::value)>.