[range.join.iterator] Rename ref-is-glvalue to account for rvalues

[JohelEGP]

We want to join rvalues (see ericniebler/stl2#598 (comment)), so the name of ref-is-glvalue is deceptive. I suggest renaming it to deref-is-ref.

[CaseyCarter]

The variable is true if and only if *i is a glvalue for a value i of type iterator_t<Base>, i.e., it's true if that iterator type's "reference type" is a reference type. (It's unfortunate that iterators have an associated type "reference type" which reuses the name of a category of core language types, doubly so given that the "reference type" of an iterator is often but not always a reference type, but that ship sailed decades ago.)

While I don't care much about the (exposition-only) name of the variable, I don't find deref-is-ref to be a substantial improvement over ref-is-glvalue. To avoid confusion - different people will interpret our cryptic abbreviations differently - I think we should pick either a short and meaningless name like potato or a long and descriptive name like has_true_reference_type or result_of_dereference_is_glvalue.

[JohelEGP]

The variable is true if and only if *i is a glvalue for a value i of type iterator_t<Base>

It is because that is not true that I opened this issue. *i could be an rvalue and the variable would be true because it uses is_reference_v and not is_lvalue_reference_v.

If ericniebler/stl2#641 was picked up, this exposition-only variable would be replaced with safe_range. In case it isn't, deref-is-ref is not deceptive even if it still uses the overloaded reference term. has_true_reference_type seems like an improvement.

[CaseyCarter]

The variable is true if and only if *i is a glvalue for a value i of type iterator_t<Base>

It is because that is not true that I opened this issue. *i could be an rvalue and the variable would be true because it uses is_reference_v and not is_lvalue_reference_v.

A glvalue is either an lvalue or an xvalue. *i is an lvalue iff is_lvalue_reference_v<decltype(*i)> is true. *i is an xvalue iff is_rvalue_reference_v<decltype(*i)> is true. *i is a glvalue iff is_reference_v<decltype(*i)> is true, so the initializer of ref-is-glvalue uses is_reference_v.

[JohelEGP]

You are right. Thank you. Unfortunately, I had already opened an LWG issue for this a while ago rather that choosing this lower barrier venue, as I prefer.