The range insertion operations of `flat_map`/`multimap`/`set`/`multiset` have this Remarks: element:

Since this operation performs an in-place merge, it may allocate memory.

It is not clear what happens if that allocation fails. If the `inplace_merge` algorithm is used, it will fall back to a less efficient 𝒪(N log N) algorithm, but this is not reflected in the Complexity: element. Alternatively, if the allocation failure is reported by an exception, that should be called out in the spec.