Proposed resolution:

This wording is relative to N4993.

1. Modify [atomics.types.operations] as indicated:

   [Drafting note: There is implementation divergence at the moment; libstdc++ already implements the proposed resolution and has done so for a while.]

   constexpr atomic() noexcept(is_nothrow_default_constructible_v<T>);
   

   -1- ConstraintsMandates: is_default_constructible_v<T> is true.

   -2- Effects: […]