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.

2567. Specification of logical operator traits uses BaseCharacteristic, which is defined only for UnaryTypeTraits and BinaryTypeTraits

Section: 21.3.9 [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 21.3.2 [meta.rqmts], but only for UnaryTypeTraits and BinaryTypeTraits. conjunction and disjunction seem to be neither.

• 21.3.2 [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 21.3.2 [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Previous resolution [SUPERSEDED]: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 a specialization 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 baseBaseCharacteristic is the last type in the list. [ Note: This means a specialization of conjunction does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

In [meta.logical]/6, edit as follows:

The BaseCharacteristic of a specialization 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 baseBaseCharacteristic is the last type in the list. [ Note: This means a specialization of disjunction does not necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

Previous resolution [SUPERSEDED]:

In [meta.logical]/3, edit as follows:

The BaseCharacteristic of a specialization 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 necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

In [meta.logical]/6, edit as follows:

The BaseCharacteristic of a specialization 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 necessarily have a BaseCharacteristic of derive from either true_type or false_type. — end note ]

Previous resolution [SUPERSEDED]:

This wording is relative to N4606.

1. In 21.3.9 [meta.logical] p3, edit as follows:

   template<class... B> struct conjunction : see below { };
   

   -3- The BaseCharacteristic of a specialization 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 of inherit from either true_type or false_type. —end note]

2. In 21.3.9 [meta.logical] p6, edit as follows:

   template<class... B> struct disjunction : see below { };
   

   -6- The BaseCharacteristic of a specialization 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 of inherit from either true_type or false_type. —end note]

3. In 21.3.9 [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.

1. In 21.3.9 [meta.logical] p3, edit as follows:

   template<class... B> struct conjunction : see below { };
   

   […]

   -3- The BaseCharacteristic of a specialization conjunction<B1, ..., BN> has a public and unambiguous base that is either

   1. — the first type Bi in the list true_type, B1, ..., BN for which bool(Bi::value) is false, or
   2. — 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 necessarily have a BaseCharacteristic of inherit 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.

2. In 21.3.9 [meta.logical] p6, edit as follows:

   template<class... B> struct disjunction : see below { };
   

   […]

   -6- The BaseCharacteristic of a specialization disjunction<B1, ..., BN> has a public and unambiguous base that is either

   1. — the first type Bi in the list false_type, B1, ..., BN for which bool(Bi::value) is true, or,
   2. — 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 necessarily have a BaseCharacteristic of inherit 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.

3. In 21.3.9 [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)>.