by partial-instantiating on Iter.

This reverts commit d6cbe96e2fc7bc8f1c10e631b52b1ef31ff9a6f5.

Isn't useful, as duplicate inheritance of same type results in ambiguity
errors...

Not enforcing it using type hierarchy.

https://github.com/harfbuzz/harfbuzz/pull/1510

We cannot rely on constexpr functions...

For stricter enforcement.

We'll see if bots like.

By specifying Item type, which is desirable.

[skip ci]

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...