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2818. "::std::" everywhere rule needs tweaking

Section: 16.4.2.2 [contents] Status: C++23 Submitter: Tim Song Opened: 2016-11-11 Last modified: 2023-11-22

Priority: 2

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Discussion:

[contents]/3 says

Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects section for library function F is described as calling library function G, the function ::std::G is meant.

With the introduction of nested namespaces inside std, this rule needs tweaking. For instance, time_point_cast's Returns clause says "time_point<Clock, ToDuration>(duration_cast<ToDuration>(t.time_since_epoch()))"; that reference to duration_cast obviously means ::std::chrono::duration_cast, not ::std::duration_cast, which doesn't exist.

[Issues Telecon 16-Dec-2016]

Priority 2; Jonathan to provide wording

[2019 Cologne Wednesday night]

Geoffrey suggested editing 16.4.2.2 [contents]/2 to mention the case when we're defining things in a sub-namespace.

Jonathan to word this.

[2020-02-14, Prague; Walter provides wording]

[2020-10-02; Issue processing telecon: new wording from Jens]

Use "Simplified suggestion" in 13 June 2020 email from Jens.

Previous resolution [SUPERSEDED]:

This wording is relative to N4849.

  1. Modify 16.4.2.2 [contents] as indicated:

    -3- Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects: element for library function F is described as calling library function G, the function ::std::G is meant.Let x be a name specified by the standard library via a declaration in namespace std or in a subnamespace of namespace std. Whenever x is used as an unqualified name in a further specification, it is assumed to correspond to the same x that would be found via unqualified name lookup (6.5.3 [basic.lookup.unqual]) performed at that point of use. Similarly, whenever x is used as a qualified name in a further specification, it is assumed to correspond to the same x that would be found via qualified name lookup (6.5.5 [basic.lookup.qual]) performed at that point of use. [Note: Such lookups can never fail in a well-formed program. — end note] [Example: If an Effects: element for a library function F specifies that library function G is to be used, the function ::std::G is intended. — end example]

Previous resolution [SUPERSEDED]:

This wording is relative to N4849.

  1. Modify 16.4.2.2 [contents] as indicated:

    [Drafting note: Consider adding a note clarifying that the unqualified lookup does not perform ADL. ]

    -3- Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects: element for library function F is described as calling library function G, the function ::std::G is meant. Whenever an unqualified name x is used in the specification of a declaration D in clauses 16-32, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D. Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of D. [Example: The reference to is_array_v in the specification of std::to_array (23.3.3.6 [array.creation]) refers to ::std::is_array_v. -- end example] [Note: Operators in expressions 12.2.2.3 [over.match.oper] are not so constrained; see 16.4.6.4 [global.functions]. -- end note]

[2020-11-04; Jens provides improved wording]

[2020-11-06; Reflector discussion]

Casey suggests to insert "or Annex D" after "in clauses 16-32". This insertion has been performed during reflector discussions immediately because it seemed editorial.

[2020-11-15; Reflector poll]

Set priority status to Tentatively Ready after seven votes in favour during reflector discussions.

[2020-11-22, Tim Song reopens]

The references to get in 25.5.4.1 [range.subrange.general] and 25.7.23.2 [range.elements.view] need to be qualified as they would otherwise refer to std::ranges::get instead of std::get. Additionally, [expos.only.func] needs to clarify that the lookup there also takes place from within namespace std.

Previous resolution [SUPERSEDED]:

This wording is relative to N4868.

  1. Modify 16.4.2.2 [contents] as indicated:

    -3- Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects: element for library function F is described as calling library function G, the function ::std::G is meant. Whenever an unqualified name x is used in the specification of a declaration D in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of D. [Example: The reference to is_array_v in the specification of std::to_array (23.3.3.6 [array.creation]) refers to ::std::is_array_v. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

  2. Remove [fs.req.namespace] in its entirety:

    29.11.3.2 Namespaces and headers [fs.req.namespace]

    -1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with ::std::filesystem::.

[2021-05-20; Jens Maurer provides an updated proposed resolution]

[2021-05-23; Daniel provides some additional tweaks to the updated proposed resolution]

[2021-05-24; Reflector poll]

Set status to Tentatively Ready after six votes in favour during reflector poll.

[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4885.

  1. Modify [expos.only.func] as indicated:

    -2- The following are defined for exposition only to aid in the specification of the library: 

    namespace std {
  template<class T> constexpr decay_t<T> decay-copy(T&& v)
      noexcept(is_nothrow_convertible_v<T, decay_t<T>>) // exposition only
    { return std::forward<T>(v); }
    
  constexpr auto synth-three-way =
    []<class T, class U>(const T& t, const U& u)
      requires requires {
      { t < u } -> boolean-testable;
      { u < t } -> boolean-testable;
      }
    {
      if constexpr (three_way_comparable_with<T, U>) {
        return t <=> u;
      } else {
        if (t < u) return weak_ordering::less;
        if (u < t) return weak_ordering::greater;
        return weak_ordering::equivalent;
      }
    };

  template<class T, class U=T>
  using synth-three-way-result = decltype(synth-three-way(declval<T&>(), declval<U&>()));
}

  2. Modify 16.4.2.2 [contents] as indicated:

    -3- Whenever a name x defined in the standard library is mentioned, the name x is assumed to be fully qualified as ::std::x, unless explicitly described otherwise. For example, if the Effects: element for library function F is described as calling library function G, the function ::std::G is meant. Whenever an unqualified name x is used in the specification of a declaration D in clauses 16-32 or Annex D, its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D. [Note ?: Argument-dependent lookup is not performed. — end note] Similarly, the meaning of a qualified-id is established as-if by performing qualified name lookup (6.5.5 [basic.lookup.qual]) in the context of D. [Example: The reference to is_array_v in the specification of std::to_array (23.3.3.6 [array.creation]) refers to ::std::is_array_v. — end example] [Note ?: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note]

  3. Modify 25.5.4.1 [range.subrange.general] as indicated:

    template<class T>
  concept pair-like =              // exposition only
    !is_reference_v<T> && requires(T t) {
      typename tuple_size<T>::type; // ensures tuple_size<T> is complete
      requires derived_from<tuple_size<T>, integral_constant<size_t, 2>>;
      typename tuple_element_t<0, remove_const_t<T>>;
      typename tuple_element_t<1, remove_const_t<T>>;
      { std::get<0>(t) } -> convertible_to<const tuple_element_t<0, T>&>;
      { std::get<1>(t) } -> convertible_to<const tuple_element_t<1, T>&>;
    };

  4. Modify 25.7.23.2 [range.elements.view] as indicated:

    template<class T, size_t N>
  concept has-tuple-element = // exposition only
    requires(T t) {
      typename tuple_size<T>::type;
      requires N <tuple_size_v<T>;
      typename tuple_element_t<N, T>;
      { std::get<N>(t) } -> convertible_to<const tuple_element_t<N, T>&>;
    };

  5. Modify 25.7.23.3 [range.elements.iterator] as indicated:

    -2- The member typedef-name iterator_category is defined if and only if Base models forward_range. In that case, iterator_category is defined as follows: […]

    1. (2.1) — If std::get<N>(*current_) is an rvalue, iterator_category denotes input_iterator_tag.

    2. […]

    static constexpr decltype(auto) get-element(const iterator_t<Base>& i); // exposition only

    -3- Effects: Equivalent to: 

    if constexpr (is_reference_v<range_reference_t<Base>>) {
  return std::get<N>(*i);
} else {
  using E = remove_cv_t<tuple_element_t<N, range_reference_t<Base>>>;
  return static_cast<E>(std::get<N>(*i));
}

  6. Remove [fs.req.namespace] in its entirety:

    29.11.3.2 Namespaces and headers [fs.req.namespace]

    -1- Unless otherwise specified, references to entities described in subclause 31.12 [filesystems] are assumed to be qualified with ::std::filesystem::.