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### 3116. `OUTERMOST_ALLOC_TRAITS` needs `remove_reference_t`

Section: 20.13.4 [allocator.adaptor.members] Status: C++20 Submitter: Tim Song Opened: 2018-06-04 Last modified: 2021-02-25

Priority: 0

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Discussion:

`OUTERMOST_ALLOC_TRAITS(x)` is currently defined in 20.13.4 [allocator.adaptor.members]p1 as `allocator_traits<decltype(OUTERMOST(x))>`. However, `OUTERMOST(x)`, as defined and used in this subclause, is an lvalue for which `decltype` produces an lvalue reference. That referenceness needs to be removed before the type can be used with `allocator_traits`.

While we are here, the current wording for `OUTERMOST` uses the imprecise "if `x` does not have an `outer_allocator()` member function". What we meant to check is the validity of the expression `x.outer_allocator()`, not whether `x` has some (possibly ambiguous and/or inaccessible) member function named `outer_allocator`.

[2018-06 Rapperswil Thursday issues processing]

Status to Ready

[2018-11, Adopted in San Diego]

Proposed resolution:

This wording is relative to N4750.

> [Drafting note: The subclause only uses `OUTERMOST_ALLOC_TRAITS(*this)` and only in non-`const` member functions, so the result is also non-`const`. Thus, `remove_reference_t` is sufficient; there's no need to further remove cv-qualification. — end drafting note]

1. Modify 20.13.4 [allocator.adaptor.members]p1 as indicated:

   > -1- In the `construct` member functions, `OUTERMOST(x)` is `x` if `x` does not have an `outer_allocator()` member function and `OUTERMOST(x.outer_allocator())` if the expression `x.outer_allocator()` is valid (13.10.3 [temp.deduct]) and `x` otherwise; `OUTERMOST_ALLOC_TRAITS(x)` is `allocator_traits<remove_reference_t<decltype(OUTERMOST(x))>>`. [Note: […] — end note]