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4265. std::midpoint should not accept const bool

Section: 26.10.16 [numeric.ops.midpoint] Status: Tentatively Ready Submitter: Jan Schultke Opened: 2025-05-21 Last modified: 2025-10-21

Priority: Not Prioritized

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Discussion:

The constraints of the first overload of std::midpoint are as follows:

template<class T>
  constexpr T midpoint(T a, T b) noexcept;

-1- Constraints: T is an arithmetic type other than bool.

It does not appear intentional that const bool is supported considering that 26.10.14 [numeric.ops.gcd] excludes cv bool.

More generally, it is questionable that cv-qualified arithmetic types aren't excluded in general, considering that 26.10.17 [numeric.sat] excludes them with the "signed or unsigned integer type" constraint. However, tightening the constraints for these other functions seems evolutionary, not like fixing an obvious oversight, and so it likely requires a separate issue.

[2025-10-21; Reflector poll.]

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

This requires template argument to be explicty specified, i.e. midpoint<const bool>. I would prefer to address all cv-qualified types at once, e.g. Constraints: remove_cv_t<T> is an arithmetic type other than bool."

"This is locally consistent with gcd and lcm which only exclude cv bool. [algorithms.requirement] p15 makes it unspecified to use an explicit template argument list here, so midpoint<const bool> and midpoint<const int> are already unspecified, this issue just ensures that const bool is explicitly rejected, like bool."

Proposed resolution:

This wording is relative to N5008.

1. Modify 26.10.16 [numeric.ops.midpoint] as indicated:

   template<class T>
     constexpr T midpoint(T a, T b) noexcept;
   

   -1- Constraints: T is an arithmetic type other than cv bool.