Implement LWG-4223 Deduction guides for maps are mishandling tuples and references

[frederick-vs-ja]

Fixes #6207. I believe the remove_cvref_t parts should be applied to C++17 mode.

[Copilot]

Pull request overview

Note

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This PR implements LWG-4223 fixes for C++ standard library deduction guides, ensuring that associative container deduction guides properly strip cv-qualifiers and references from key and value types.

Changes:

• Updates _Guide_key_t, _Guide_val_t, _Guide_pair_t, _Range_key_type, _Range_mapped_type, and _Range_to_alloc_type to use remove_cvref_t/_Remove_cvref_t and tuple_element_t instead of direct member access with only remove_const_t.
• Adds comprehensive tests for map/multimap and unordered_map/unordered_multimap deduction with cv-ref qualified pair/tuple types.
• Updates flat_map tests to verify LWG-4223 with an almost_pair type that no longer exposes first_type/second_type aliases.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
stl/inc/xutility	Updates _Guide_key_t, _Guide_val_t to use remove_cvref_t and simplifies _Guide_pair_t
stl/inc/xmemory	Updates _Range_key_type, _Range_mapped_type, _Range_to_alloc_type to use tuple_element_t with remove_cvref_t
tests/std/tests/P0433R2_deduction_guides/test.cpp	Adds deduction guide tests for pairs/tuples with cv-ref qualified types
tests/std/tests/P0429R9_flat_map/test.cpp	Updates almost_pair and adds flat_map/flat_multimap construction tests for LWG-4223

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