[libcxx-commits] [libcxx] [libc++] Resolve LWG4308, correct `iterator` availability for `optional<T&>` (PR #173948)

[William Tran-Viet via libcxx-commits]

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smallp-o-p wrote:

Test changes in another PR (which will come up pretty much right after this gets merged in), the nit is related to this one.

https://github.com/llvm/llvm-project/pull/173948