...Ah, `heap_sort` is used only for trivially copyable types. So my complaint about not distinguishing copy from move is essentially unimportant (matters only in pathological cases that we shouldn't worry about).

But it's perfectly possible for a type to be "trivially copyable" without being "default-constructible." An example of such a type from the STL: `std::reference_wrapper<int>`.

Anyway, looks like a quick fix for this would be to just extend the list of traits on which blqsort is gated (currently `is_trivially_copyable` and `sizeof(T) <= 16`) by adding `is_trivially_default_constructible<T>::value` also.