Issue 2567: Specification of logical operator traits uses BaseCharacteristic, which is defined only for UnaryTypeTraits

and BinaryTypeTraits (original) (raw)

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

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Tim Song Opened: 2015-12-10 Last modified: 2017-07-30

Priority: 2

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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(i) and 2568(i)

Previous resolution [SUPERSEDED]:

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

The BaseCharacteristic of a specialization conjunction<B1, ..., BN> has a public and unambiguous base thatis the first type Bi in the list true_type, B1, ..., BN for which Bi::value == false, or if every Bi::value != false, theaforementioned 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 thatis the first type Bi in the list false_type, B1, ..., BN for which Bi::value != false, or if every Bi::value == false, theaforementioned 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 ]

Merged the resolution of [2587](lwg-defects.html#2587 ""Convertible to bool" requirement in conjunction and disjunction (Status: C++17)")(i) 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.

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 struct negation : bool_constant<!bool(B::value)> { };

   -8- The class template negation forms the logical negation of its template type argument. The typenegation<B> is a UnaryTypeTrait with a BaseCharacteristic ofbool_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 struct negation : see belowbool_constant<!B::value> { };

   -8- The class template negation forms the logical negation of its template type argument. The typenegation<B> is a UnaryTypeTrait with a BaseCharacteristic ofbool_constant<!bool(B::value)>.