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2557. Logical operator traits are broken in the zero-argument case

Section: 21.3.9 [meta.logical] Status: C++17 Submitter: Geoffrey Romer Opened: 2015-11-05 Last modified: 2017-07-30

Priority: 0

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Discussion:

The conjunction trait in 21.3.9 [meta.logical] seems intended to support invocation with zero arguments, e.g. conjunction<>::value, which is likely to be a useful feature. However, the specification doesn't actually make sense in the zero-argument case. See 21.3.9 [meta.logical]/p3:

The BaseCharacteristic of a specialization conjunction<B1, …, BN> 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 BN.

If "B1, ..., BN" is an empty list, then every B::value != false, so the BaseCharacteristic is BN, but there is no BN in this case.

(If LWG concludes that conjunction intentionally requires at least one argument, I would appreciate their confirmation that I can editorially remove the mention of true_type, which seems to have no normative impact outside the zero-argument case.)

Similar comments apply to the disjunction trait, and to the corresponding traits in the Fundamentals working paper, see LWG 2558(i).

[2016-02, Issues Telecon]

P0; move to Tentatively Ready.

Proposed resolution:

This wording is relative to N4567.

  1. Revise 21.3.9 [meta.logical] as follows:

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

    […]

    -3- The BaseCharacteristic of a specialization conjunction<B1, ..., BN> 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 BNthe last type in the list. [Note: This means a specialization of conjunction does not necessarily have a BaseCharacteristic of either true_type or false_type. — end note]

    […] 

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

    […]

    -6- The BaseCharacteristic of a specialization disjunction<B1, ..., BN> 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 BNthe last type in the list. [Note: This means a specialization of disjunction does not necessarily have a BaseCharacteristic of either true_type or false_type. — end note]

    […]