Issue 3982: is-derived-from-view-interface should require that T is derived from view

3982. `is-derived-from-view-interface` should require that `T` is derived from `view_interface<T>`

Section: 25.4.4 [range.view] Status: Tentatively NAD Submitter: Hewill Kang Opened: 2023-08-22 Last modified: 2023-10-30

Priority: Not Prioritized

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Discussion:

Currently, the wording of is-derived-from-view-interface only detects whether type T is unambiguously derived from one base class view_interface<U> where U is not required to be T, which is not the intention of CRTP.

[2023-10-30; Reflector poll]

Set status to Tentatively NAD. The wording correctly handles the case where T derives from Base which derives from view_interface<Base>. We don't want it to only be satisfied for direct inheritance from view_interface<T>, but from any specialization of view_interface. Previously the concept only checked for inheritance from view_base but it was changed when view_interface stopped inheriting from view_base.

Proposed resolution:

This wording is relative to N4958.

Modify 25.4.4 [range.view] as indicated:
```
template<class T>
  constexpr bool is-derived-from-view-interface = see below;            // exposition only
template<class T>
  constexpr bool enable_view =
    derived_from<T, view_base> || is-derived-from-view-interface<T>;
```
-6- For a type T, is-derived-from-view-interface<T> is true if and only if T has exactly one public base class view_interface<TU> ~~for some type U~~ and T has no base classes of type view_interface<UV> for any other type UV.

3982. is-derived-from-view-interface should require that T is derived from view_interface<T>

3982. `is-derived-from-view-interface` should require that `T` is derived from `view_interface<T>`