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### 3649. [fund.ts.v3] Reinstate and bump `__cpp_lib_experimental_memory_resource` feature test macro

Section: 1.5 [fund.ts.v3::general.feature.test] Status: Tentatively Ready Submitter: Thomas Köppe Opened: 2021-12-03 Last modified: 2022-02-11

Priority: Not Prioritized

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Discussion:

Addresses: fund.ts.v3

The rebase on C++17 in P0996 had the effect of deleting the feature test macro `__cpp_lib_experimental_memory_resource`: the macro was ostensibly tied to the `memory_resource` facility that had become part of C++17, but we overlooked that there was a residual piece that has not been adopted in the IS, namely the `resource_adaptor` class template.

It is still useful to be able to detect the presence of `resource_adaptor`, so we should reinstate the feature test macro and bump its value.

[2022-01-30; Reflector poll]

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

Proposed resolution:

This wording is relative to N4853.

1. Modify 1.5 [fund.ts.v3::general.feature.test], Table 2, as indicated:

   > Table 2 — Significant features in this technical specification
   > Doc. No. | Title | Primary Section | Macro Name Suffix | Value | Header
   > --- | --- | --- | --- | --- | ---
   > `…`
   > N3916 | Type-erased allocator for std::function | 4.2 | `function_erased_allocator` | `201406` | `<experimental/functional>`
   > N3916 | Polymorphic Memory Resources | 5.4 [fund.ts.v3::memory.resource.syn] | `memory_resources` | [new value] | `<experimental/memory_resource>`
   > N4282 | The World's Dumbest Smart Pointer | 8.12 | `observer_ptr` | `201411` | `<experimental/memory>`
   > `…`