RFC for zip_view implementation, for oneDPL C++20

rfcs/proposed/zip_view/README.md

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+# zip_view Support for the oneDPL Range APIs with C++20
+
+We propose to add `ranges::zip_view` to oneDPL, with at least the same API and functionality as `std::ranges::zip_view`.
+
+## Motivation
+`std::ranges::zip_view` is a convenient way to combine multiple ranges into a single view, where each element of
+the resulting range is a tuple referring to corresponding elements from each of the input ranges. This can be particularly
+useful for iterating over multiple collections in parallel. `std::ranges::zip_view` is added in C++23,
+but many developers are still using the C++20 standard, and it is also the required standard for oneDPL parallel range algorithms.
+
+In case of C++23 having a `zip_view` in oneDPL also makes sense at least for the device policies, because
+`std::ranges::zip_view` still is not device copyable (any wrapper over `std::tuple` is not device copyable, see https://godbolt.org/z/brfvcMeM6)
+There are other technical issues with `std::tuple` (see below for the details).
+
+## Key Requirements
+`ranges::zip_view` should be:
+- compilable with C++20 version (minimum);
+- API-compliant with `std::ranges::zip_view`;
+- in case of device usage: a device copyable view if all underlying views are device copyable views;
+- The implementation may be based on tuple-like type underhood, but it must provide transitive device copyability
+  for view pipelines created over `ranges::zip_view`.
+
+`ranges::zip_view::iterator` should be:
+- value-swappable (https://en.cppreference.com/w/cpp/named_req/ValueSwappable)
+- indirectly writable (https://en.cppreference.com/w/cpp/iterator/indirectly_writable.html)
+- able to be used with the non-range algorithms, including C++ and oneDPL parallel algorithms:
+  depending on the algorithm, stricter requirements may apply. For example, `std::sortable` concept must be satisfied in order to call `std::ranges::sort`.
+
+### Discrepancies with std::zip_view C++23
+- `ranges::zip_view` may use a oneDPL tuple-like type instead of `std::tuple` 
+- `ranges::zip_view::iterator::value_type` should not be defined as `std::tuple` (see a known technical issue below)
+
+### Other technical reasons not to use std::zip_view C++23 (and std::tuple) with oneDPL algorithms in the future:
+- There is an issue with `std::ranges::sort(zip_view)` with clang 19.0 and older. (https://godbolt.org/z/jKvG9rY5M)
+- There is an issue with `std::ranges::stable_sort(zip_view)` with gcc library 
+- Passing `std::zip_view::iterator` instances to the iterator-based algorithms works only for gcc 14.1 and newer, clang 19.1 and newer or
+  starting 17.01 with libc++ lib (https://godbolt.org/z/To6Mjr9M6)
+- Consideration `std::tuple` as `ranges::zip_view::iterator::value_type`. There are issues, at least, with `sortable`, `permutable`
+  and `indirectly_writable` concepts: `const_cast<const std::iter_reference_t<Out>&&>(*o) = std::forward<T>(t)` is not compiled till C++23.  (https://godbolt.org/z/zT9qqnjWq)
+
+## Implementation proposal (C++20)
+- `ranges::zip_view` is as a C++ class representing a range adaptor (see C++ Range Library).
+- The implementation derives from the C++ `std::ranges::view_interface`.
+This class encapsulates a tuple-like type to keep a combination of two or more ranges.
+- To ensure device copyability, `oneapi::dpl::__internal::tuple` is proposed as a tuple-like type for underlying elements.
+- To provide a value-swappable requirement `oneapi::dpl::__internal::tuple` is proposed as a dereferenced value for
+`ranges::zip_view::iterator` due to `std::tuple` not satisfying the value-swappable requirement in C++20.
+- To provide an indirectly writable requirement `oneapi::dpl::__internal::tuple` is proposed as the public type for `ranges::zip_view::iterator::value_type`.
+- Usage of C++ concepts is desirable to write type requirements for types, methods and members of the class.
+- C++20 is minimum supported version for the class. It allows using modern C++ features such as concepts and others.
+- A range adapter `zip_view` is available in the `oneapi::dpl::experimental::ranges` namespace.
+- A customization point object `zip` is available in the `oneapi::dpl::experimental::ranges::views` and `oneapi::dpl::experimental::views` namespaces.
+
+### Test coverage
+- `ranges::zip_view` is tested itself, base functionality (the API that is used for a range in the oneDPL algorithm implementations)
+- the base functionality test coverage may be extended by the adapted LLVM `std::ranges::zip_view` (C++23) tests.
+- should be tested with range based algorithms.
+- should be tested with iterator based algorithms.
+
+### Experimental stage
+Until the API is fully specified, it should go into `namespace oneapi::dpl::experimental`.
+For C++20 and higher, it replaces the current implementation in that namespace, extending applicability
+beyond experimental range algorithms but still supporting those.