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4312. Const and value category mismatch for allocator_arg_t/allocator_arg in the description of uses-allocator construction

Section: 20.2.8.2 [allocator.uses.construction] Status: Tentatively Ready Submitter: Jiang An Opened: 2025-08-06 Last modified: 2025-10-14

Priority: Not Prioritized

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Discussion:

Currently, 20.2.8.2 [allocator.uses.construction] bullet 2.2 states:

Otherwise, if T has a constructor invocable as T(allocator_arg, alloc, args...) (leading-allocator convention), […]

However, when forming construction arguments in the utility functions, we're actually using cv-unqualified rvalue of allocator_arg_t, which can be inferred from using plain allocator_arg_t but not const allocator_arg_t& in 20.2.8.2 [allocator.uses.construction] bullet 5.2.

It seems that such mismatch was present even since C++11 (per N3337 [allocator.uses.construction]/1.2). If the use of plain allocator_arg_t is considered correct, I think we should fix the description.

[2025-10-14; Reflector poll]

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

Unless the std::allocator_arg tag object is not supposed to be used, wouldn't it make more sense to preserve the "if T has a constructor invocable as T(allocator_arg, alloc, args...)" wording and change every allocator_arg_t into const allocator_arg_t&, so that we check for construction from the const tag object, and then actually use a const value in the constructor arguments. Strongly don't care though.

Proposed resolution:

This wording is relative to N5014.

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

   -2- The following utility functions support three conventions for passing alloc to a constructor:

   1. (2.1) — […]

   2. (2.2) — Otherwise, if T has a constructor invocable as T(allocator_argallocator_arg_t{}, alloc, args...) (leading-allocator convention), then uses-allocator construction chooses this constructor form.

   3. (2.3) — […]