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3690. std::make_from_tuple etc. should find all tuple-like std::get overloads

Section: 16.4.2.2 [contents], 22.4.6 [tuple.apply] Status: New Submitter: Jiang An Opened: 2022-04-06 Last modified: 2022-05-17

Priority: 3

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Discussion:

Currently it is not clear in 16.4.2.2 [contents]/3 whether all possible overloads in the standard library are considered to be found "in the context of D". As a result, it seems underspecified whether a certain std::get overload is found by std::tuple_cat, std::make_from_tuple, std::apply, or exposition-only concept pair-like or has-tuple-element.

There is implementation divergence: MSVC STL's std::make_from_tuple accepts std::ranges::subrange, but libstdc++'s doesn't, which is originally discussed in GCC bug #102301.

IMO std::get overloads need some special rules: when referred by tuple-like facilities, overloads for std::variant should be excluded (or at least leave whether it's found unspecified), and all other overloads should be found; and the opposite rule should be used when referred in 22.6 [variant].

[2022-04-25; Jiang An comments and provides wording]

Currently this program is accepted when using MSVC STL and libstdc++, although the acception seems unintended and problematic. 

#include <variant>
#include <span>
#include <ranges>

struct Foo : std::variant<int, long> {};

template<>
struct std::tuple_element<0, Foo> { using type = int; };

template<>
struct std::tuple_element<1, Foo> { using type = long; };

template<>
struct std::tuple_size<Foo> : std::integral_constant<std::size_t, 2> {};

constexpr auto bad_keys = std::span<Foo>{} | std::views::values;

int main() {} // COMPILE-ONLY

[2022-05-17; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4910.

  1. Modify 16.4.2.2 [contents] as indicated:

    […]

    -3- Whenever an unqualified name other than swap is used in the specification of a declaration D in Clause 17 [support] through Clause 32 [thread] or Annex D [depr], its meaning is established as-if by performing unqualified name lookup (6.5.3 [basic.lookup.unqual]) in the context of D.

    [Note 1: 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 1: 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 2: Operators in expressions (12.2.2.3 [over.match.oper]) are not so constrained; see 16.4.6.4 [global.functions]. — end note] The meaning of the unqualified name swap is established in an overload resolution context for swappable values (16.4.4.3 [swappable.requirements]).

    Certain entities in the standard library are specified to select tuple-like get function templates. An implementation shall behave as if every tuple-like get function template is found in the definition of such an entity. Furthermore, an implementation shall ensure that no get function template that is not tuple-like is found in the definition of such an entity.

  2. Add to the end of 22.3.4 [pair.astuple], 22.4.8 [tuple.elem], 23.3.3.7 [array.tuple], and 25.5.4.3 [range.subrange.access] as indicated:

    The get function templates specified in this section are tuple-like (16.4.2.2 [contents]).

  3. Modify 22.4.6 [tuple.apply] as indicated:

    template<class F, class Tuple>
  constexpr decltype(auto) apply(F&& f, Tuple&& t);

    -1- Effects: Given the exposition-only function: 

    namespace std {
  template<class F, class Tuple, size_t... I>
  constexpr decltype(auto) apply-impl(F&& f, Tuple&& t, index_sequence<I...>) { // exposition only
    return INVOKE(std::forward<F>(f), get<I>(std::forward<Tuple>(t))...); // see 22.10.4 [func.require]
  }
}

    Equivalent to: 

    return apply-impl(std::forward<F>(f), std::forward<Tuple>(t),
                  make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

    -?- Remarks: apply-impl selects tuple-like get function templates. 

    template<class T, class Tuple>
  constexpr T make_from_tuple(Tuple&& t);

    -2- Mandates: If tuple_size_v<remove_reference_t<Tuple>> is 1, then reference_constructs_from_temporary_v<T, decltype(get<0>(declval<Tuple>()))> is false.

    -3- Effects: Given the exposition-only function: 

    namespace std {
  template<class T, class Tuple, size_t... I>
    requires is_constructible_v<T, decltype(get<I>(declval<Tuple>()))...>
  constexpr T make-from-tuple-impl(Tuple&& t, index_sequence<I...>) { // exposition only
    return T(get<I>(std::forward<Tuple>(t))...);
  }
}

    Equivalent to: 

    return make-from-tuple-impl<T>(
          std::forward<Tuple>(t),
          make_index_sequence<tuple_size_v<remove_reference_t<Tuple>>>{});

    […]

    -?- Remarks: make-from-tuple-impl selects tuple-like get function templates.

  4. Add at the end of 25.5.4.1 [range.subrange.general] (after the synopsis) as indicated:

    [Drafting note: Although IIUC pair-like is not needed to handle array and subrange.]

    […]

    -?- Remarks: pair-like selects tuple-like get function templates.

  5. Add after the synopsis of 25.7.23.2 [range.elements.view] as indicated:

    […]

    -?- Remarks: has-tuple-element selects tuple-like get function templates.

    […]