Implement LWG-3797 elements_view insufficiently constrained

[frederick-vs-ja]

Fixes #6204.

However, it seems that elements_view becomes overconstrained with this fix, although it was already overconstrained in C++20. Currently (after LWG-3797) the following code is rejected, while it seems to me that it can be made to work.

using MoveOnlySubrange = std::ranges::subrange<MoveOnlyIter, Sentinel>;
std::vector<MoveOnlySubrange> v;
for (auto&& elem : v | std::views::as_rvalue | std::views::elements<0>) {
    // ...
}

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Seems like that the constraints of elements_view are somehow imprecise. Especially, _Has_tuple_element doesn't seem to handle rvalues well. IMO, ideally, they should only match validness of iterator's operator* while additionally requiring tuple-like-ness.

[Copilot]

Pull request overview

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This PR implements LWG-3797 "elements_view insufficiently constrained" by adding a missing constraint to the _Has_tuple_element concept in C++23 mode, ensuring that get<N>(t) is convertible to const tuple_element_t<N, T>&. The pre-C++23 path already had this constraint, but it was inadvertently dropped when the C++23 _Tuple_like concept was introduced.

Changes:

• Added the get<_Index>(__t) -> convertible_to<const tuple_element_t<_Index, _Tuple>&> constraint to the C++23 branch of _Has_tuple_element.
• Added compile-only tests verifying that elements_view correctly rejects ranges whose element type has a move-only component (e.g., subranges with move-only input iterators), covering direct vectors, transformed views, as_const, and as_rvalue.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
stl/inc/ranges	Adds the missing requires constraint for get convertibility to _Has_tuple_element in C++23 mode
tests/std/tests/P0896R4_views_elements/test.cpp	Adds compile-time tests for LWG-3797 using move-only subranges and various view adaptors

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