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3627. Inconsistent specifications for std::make_optional overloads

Section: 22.5.9 [optional.specalg] Status: New Submitter: Jiang An Opened: 2021-10-23 Last modified: 2022-01-29

Priority: 3

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Three std::make_optional overloads are specified in 22.5.9 [optional.specalg]. The first one is specified by "Returns:" and the other two are specified by "Effects: Equivalent to:". According to [structure.specifications]/4, such uses of "Effects: Equivalent to:" propagate the Constraints specified for constructors. As the selected constructor for the first overload has "Constraints:" ( [optional.ctor]/22), it seems that inconsistency is introduced here.

Existing implementations are inconsistent: libstdc++ constrains all three overloads, while libc++ and MSVC STL do not constrain any of them.

IMO all three overloads should be constrained.

[2022-01-29; Reflector poll]

Set priority to 3 after reflector poll.

Proposed resolution:

This wording is relative to N4901.

  1. Modify 22.5.9 [optional.specalg] as indicated:

    template<class T> constexpr optional<decay_t<T>> make_optional(T&& v);

    -3- ReturnsEffects: Equivalent to: return optional<decay_t<T>>(std::forward<T>(v));.