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### 3256. Feature testing macro for `constexpr` algorithms

Section: 17.3.1 [support.limits.general] Status: C++20 Submitter: Antony Polukhin Opened: 2019-08-14 Last modified: 2021-02-25

Priority: 0

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Discussion:

Feature testing macro from P0202 "Add Constexpr Modifiers to Functions in `<algorithm>` and `<utility>` Headers" is missing in the WD.

For user convenience and to reduce feature testing macro count it would be better to stick to initial version of P0202 that was providing only the `__cpp_lib_constexpr_algorithms`.

So remove `__cpp_lib_constexpr_swap_algorithms`, define `__cpp_lib_constexpr_algorithms` to `201703L` if P0202 is implemented, to `201806L` if P0202+P0879 are implemented.

[2019-09-02 Issue Prioritization]

Status to Tentatively Ready and priority to 0 after five positive votes on the reflector.

Proposed resolution:

This wording is relative to N4830.

1. Modify the Table 36 "Standard library feature-test macros" [tab:support.ft] in 17.3.1 [support.limits.general] as indicated:

   > Table 36: Standard library feature-test macros [tab:support.ft]
   > Macro name | Value | Header(s)
   > --- | --- | ---
   > `[…]`
   > `__cpp_lib_constexpr_swap_algorithms` | `201806L` | `<algorithm>`
   > `[…]`