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3527. uses_allocator_construction_args handles rvalue pairs of rvalue references incorrectly

Section: 20.2.8.2 [allocator.uses.construction] Status: WP Submitter: Tim Song Opened: 2021-02-27 Last modified: 2021-06-07

Priority: Not Prioritized

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Discussion:

For an rvalue pair pr, uses_allocator_construction_args is specified to forward std::move(pr).first and std::move(pr).second. This is correct for non-references and lvalue references, but wrong for rvalue refrences because the class member access produces an lvalue (see 7.6.1.5 [expr.ref]/6). get produces an xvalue, which is what is desired here.

[2021-03-12; Reflector poll]

Set status to Tentatively Ready after five 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 N4878.

1. Edit 20.2.8.2 [allocator.uses.construction] as indicated:

   template<class T, class Alloc, class U, class V>
     constexpr auto uses_allocator_construction_args(const Alloc& alloc,
                                                     pair<U, V>&& pr) noexcept -> see below;
   

   […]

   -15- Effects: Equivalent to:

   return uses_allocator_construction_args<T>(alloc, piecewise_construct,
                                               forward_as_tuple(std::move(pr).firstget<0>(std::move(pr))),
                                               forward_as_tuple(std::move(pr).secondget<1>(std::move(pr))));