Implement LWG-4125 move_iterator's default constructor should be constrained

[frederick-vs-ja]

Fixes #6314. Fixes #3377 (by making LLVM-60293 and VSO-1900294 no longer relevant).

Unblocked libcxx test(s):

• std/iterators/predef.iterators/move.iterators/move.iterator/iterator_concept_conformance.compile.pass.cpp

Remarks:

• The associated constraint is added since C++20, which is consistent with WG21-P2325R3 that is clearly a DR against C++20.
• This PR also uses the default member initializer = _Iter(); in C++14 mode. Due to lack of guaranteed copy elision in C++14, such default member initializer would cause one more move construction when RVO is not performed. The move construction is required to be well-formed, and in most (but not all) situations it won't cause side effects.
• Pre-existing default member initializer {} accepts invalid cases. E.g. empty_list_input_iter added to the test file can be direct-list-initialized from {}, but can't be value initialized. So the default constructor of empty_list_input_iter should be ill-formed in old modes.

[Copilot]

Pull request overview

This PR implements the LWG-4125 resolution, which constrains move_iterator's default constructor so it only participates when the underlying iterator type is default-initializable. This also resolves the long-standing Clang/EDG divergence described in #3377 (LLVM-60293) by ensuring the default constructor is genuinely absent — rather than present-but-deleted via a default member initializer — for non-default-initializable iterators.

Changes:

• In <xutility>, move_iterator's defaulted default constructor gains a requires default_initializable<_Iter> constraint (C++20), and the _Current member initializer changes from list-init {} to value-init _Iter() (C++17) to avoid wrongly selecting an initializer_list constructor; in C++14 the default constructor becomes user-provided (: _Current()) and intentionally non-constexpr.
• Adds tests for the new constraint, including a new empty_list_input_iter helper that is initializable via {} but not value-initializable.
• Removes the now-unnecessary __clang__/__EDG__ workaround in the join_with test that previously guarded the range-of-rvalue delimiter case.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
stl/inc/xutility	Constrains move_iterator() with default_initializable<_Iter> (C++20), switches the member initializer to value-init, and restructures the constructor across C++14/17/20 modes.
tests/std/tests/P0896R4_ranges_iterator_machinery/test.cpp	Adds empty_list_input_iter and static_asserts validating default_initializable<move_iterator<...>> and iterator-category metaprogramming.
tests/std/tests/P2441R2_views_join_with/test.cpp	Removes the LLVM-60293/VSO-1900294 workaround now that the root cause is fixed.

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[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.