Additional note (August, 2022):

If there are concerns about reuse of tail padding in #1, requiring a copy for some implementation reason, similar concerns should apply to #2 if the data member is declared with [[no_unique_address]].

Furthermore, the following example using a delegating constructor shows implementation divergence:

struct Noncopyable {
  Noncopyable();
  Noncopyable(const Noncopyable &) = delete;
  Noncopyable(int) : Noncopyable(Noncopyable()) {} // #3?
};

Given the following example,

  struct Noncopyable {
    Noncopyable();
    Noncopyable(const Noncopyable &) = delete;
  };

  Noncopyable make(int kind = 0);

  struct AsBase : Noncopyable {
    AsBase() : Noncopyable(make()) {} // #1
  };

  struct AsMember {
    Noncopyable nc;
    AsMember() : nc(make()) { }  // #2?
  };

All implementations treat #1 as an error, invoking the deleted copy constructor, while #2 is accepted. It's not clear from the current wording why they should be treated differently.