Proposed resolution:

This wording is relative to N4901.

1. Modify [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));.

[ 2022-01-29; Reflector poll ]

Set priority to 3 after reflector poll.

Three std::make_optional overloads are specified in [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.