The typical use-case of std::initializer_list<T> is for a pass-by-value parameter of T's constructor. However, this contravenes [res.on.functions]/2.5 because initializer_list doesn't specifically allow incomplete types (as do for example std::unique_ptr ([unique.ptr]/5) and std::enable_shared_from_this ([util.smartptr.enab]/2)).

A resolution would be to copy-paste the relevant text from such a paragraph.