Implement LWG-4218 Constraint recursion in basic_const_iterator's relational operators due to ADL + CWG-2369

[frederick-vs-ja]

Fixes #6316.

Remarks: Currently, MSVC doesn't give constraint recursion for the original example posted in LWG-4218, perhaps because MSVC hasn't completely implemented CWG-2369. In addition to the original example, this PR adds another example causing constraint recursion for both MSVC and Clang when LWG-4218 is not implemented.

Reported DevCom-11111696 for a bug encountered in CI (with workaround added).

[Copilot]

Pull request overview

This PR implements the LWG-4218 resolution to fix constraint recursion in basic_const_iterator's reversed relational operators (<, >, <=, >=) that occurs due to ADL combined with CWG-2369. The fix lives in the core <xutility> header that defines basic_const_iterator, and is accompanied by regression tests.

Changes:

• Add a deduced, same_as<_Iter>-constrained template parameter _Jter to the four reversed relational friend operators and change their basic_const_iterator parameter to basic_const_iterator<_Jter>, breaking the constraint-satisfaction recursion.
• Add compile-time static_assert tests covering both a heterogeneous-comparison ADL scenario and a reverse_iterator<basic_const_iterator<...>> scenario.

Reviewed changes

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

File	Description
stl/inc/xutility	Makes the four reversed relational friend operators' right-hand operand depend on a deduced same_as<_Iter> parameter to avoid the LWG-4218 ADL constraint recursion.
tests/std/tests/P2278R4_basic_const_iterator/test.cpp	Adds an lwg_4218 namespace with static_asserts that would fail to compile (constraint recursion) without the fix.

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