We cannot rely on constexpr functions...

For stricter enforcement.

We'll see if bots like.

By specifying Item type, which is desirable.

Too ugly to remove..

To see if it makes bots happy... This is frustrating.

That's a C++11 extension apparently...

Removes use of auto type deduction again, which was not supported on many bots.

Not all C++11 features are created equal when it comes to support...

lvalue iterators must declare __item_type__ as a reference.

Ugly, but does the job.

Still not satisfied with how I can enforce iterators only, but
seems to work for now.