C++23 insert_range and std::from_range_t constructors

[neversleeping]

unordered containers currently have no C++23 insert_range method or constructors with std::from_range_t. These features don't look like something completely incompatible with these containers, so maybe they can be added?

[cmazakas]

We can probably even add some of these tag types to Compat so these overloads can be back-ported when they're not available. Thoughts, @joaquintides?

[joaquintides]

We can probably even add some of these tag types to Compat[...]

Yes, absolutely. Proposed boost::container::hub already defines a fallback type for that, so it makes sense that the thing is refactored into Compat.

[pdimov]

Using Compat types in interfaces creates problems, which is why it's discouraged. We can either make compat::from_range_t an alias to std::from_range_t when it exists, or not. In the former case, the interface of the unordered containers will now change depending on whether std::from_range_t exists, which can lead to ODR violations when translation units using different -std= are linked together.

In the latter case, we'll still have to add a separate, guarded, overload taking std::from_range_t in addition to the one taking compat::from_range_t.

[pdimov]

If this is the case, boost::container::from_range_t is equal to C++23 std::from_range_t if this is provided; otherwise, it is a different type with the same characteristics.

This has the same ODR problems as the first alternative above.

[cmazakas]

The ODR considerations are good. Without having implemented it,

In the latter case, we'll still have to add a separate, guarded, overload taking std::from_range_t in addition to the one taking compat::from_range_t.

This seems like it should work and avoid those aforementioned problems.

[joaquintides]

Yes, we can even have something like this for minimum code duplication:

template<
  /* container-compatible-range<T> */ R,
  /* converts-to-std::from_range_t-or-compat::from_range_t */ FromRangeT
>
unordered_flat_map(FromRangeT&&, R&& rg, const Allocator& = Allocator());