[range.cartesian.view] Fix _`cartesian-product-is-common`_

[JohelEGP]

What is Const? Why is Vs... unused? The refined concept only takes 1 argument.

Presumably, Const came from a copy-pasted maybe-const<Const, First>. That'd explain the extra closing >.

Considering the name of the concept and how it's used, this should be:

    (@\exposconcept{cartesian-product-common-arg}@<First> && ... &&
     @\exposconcept{cartesian-product-common-arg}@<Vs>);

Originally posted by @JohelEGP in #5647 (comment)

[tkoeppe]

I think you're right, will send a PR.

[hewillk]

Is this really correct? From the original paper:

cartesian_product_view can be a common range if the first range either is common, or is sized and random access. This paper reflects this.

In my opinion this should be

template <class First/*, class... Vs*/>
  concept cartesian-product-is-common = // exposition only
    cartesian-product-common-arg<First>;

@tkoeppe @JohelEGP

[JohelEGP]

Then I'd also question why the end() overloads are constrained with @\exposconcept{cartesian-product-is-common}@<First, Vs...> rather than cartesian-product-common-arg<First> directly.

[hewillk]

cartesian-product-common-arg might not be a good constraint name for end, cartesian-product-is-common seems to be additionally defined for consistency with cartesian-product-is-sized.

[tkoeppe]

@griwes: ^^^ (This was the original issue.)

[griwes]

The reason why common-arg isn't used directly in the constraint is that it's a "lower level" helper concept that's also used elsewhere, whereas is-common is a "higher level" helper concept that's used specifically for answering the question, "is a cartesian product of all of these ranges a common range?". That's also why it takes all the ranges as arguments, instead of just the first one.