[libcxx-commits] [libcxx] [libc++] Introduce `__product_iterator_traits` and optimise `flat_map::insert` (PR #139454)

[Louis Dionne via libcxx-commits]

https://github.com/ldionne requested changes to this pull request.

This looks great, thanks for the optimization! A few comments but this looks mostly good.

https://github.com/llvm/llvm-project/pull/139454