This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of TS status.

### 2568. [fund.ts.v2] Specification of logical operator traits uses `BaseCharacteristic`, which is defined only for `UnaryTypeTraits` and `BinaryTypeTraits`

Section: 3.3.3 [fund.ts.v2::meta.logical] Status: TS Submitter: Tim Song Opened: 2015-12-10 Last modified: 2020-09-06

Priority: 2

View all other issues in [fund.ts.v2::meta.logical].

View all issues with TS status.

Discussion:

Addresses: fund.ts.v2

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.

[2016-08-07 Daniel provides wording borrowed from 2567]

[2016-11-08, Issaquah]

Adopted during NB comment resolution

Proposed resolution:

This wording is relative to N4600.

1. In 3.3.3 [fund.ts.v2::meta.logical] p3, edit as follows:

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

   > > -2- The class template `conjunction` forms the logical conjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data member `value` which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

   > > -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 `B` in the list `true_type, B1, …, BN` for which `B::value == false`, or if `every B::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 3.3.3 [fund.ts.v2::meta.logical] p6, edit as follows:

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

   > > -5- The class template `disjunction` forms the logical disjunction of its template type arguments. Every template type argument shall be usable as a base class and shall have a static data member `value` which is convertible to `bool`, is not hidden, and is unambiguously available in the type.

   > > -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 `B` in the list `false_type, B1, …, BN` for which `B::value != false`, or if `every B::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 3.3.3 [fund.ts.v2::meta.logical] p8, edit as follows:

   >     template<class B> struct negation : integral_constant<bool, !B::value>see below { };

   > > -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 `integral_constant<bool, !bool(B::value)>`.