Proposed resolution:

This wording is relative to N4618.

1. Edit [optional.ctor] as indicated:

   template <class U = T>
     EXPLICIT constexpr optional(U&& v);
   

   […]

   -22- Remarks: If T's selected constructor is a constexpr constructor, this constructor shall be a constexpr constructor. This constructor shall not participate in overload resolution unless is_constructible_v<T, U&&> is true, is_same_v<decay_t<U>, in_place_t> is false, and is_same_v<optional<T>, decay_t<U>> is false. The constructor is explicit if and only if is_convertible_v<U&&, T> is false.