Proposed resolution:

This wording is relative to N5032.

1. Modify [flat.set.modifiers] as indicated:

   template<container-compatible-range<value_type> R>
     constexpr void insert_range(R&& rg);
   

   -10- Effects: Adds elements to c as if by:

   ranges::for_each(rg, [&](value_typeauto&& e) {
     c.insert(c.end(), std::movestd::forward<decltype(e)>(e));
   });
   

2. Modify [flat.multiset.modifiers] as indicated:

   template<container-compatible-range<value_type> R>
     void insert_range(R&& rg);
   

   -9- Effects: Adds elements to c as if by:

   ranges::for_each(rg, [&](value_typeauto&& e) {
     c.insert(c.end(), std::movestd::forward<decltype(e)>(e));
   });
   

[ 2026-02-20; LWG telecon; Status changed: New → Ready. ]

Implementations can optimize away the extra move for the case where the range's value type is already the same as the flat set's value type.

[ 2026-02-18; Reflector poll. ]

Set priority to 2 after reflector poll.

Pessimizing a very common case (matching type), for uncommon one.

The function adds elements via:

ranges::for_each(rg, [&](auto&& e) {
  c.insert(c.end(), std::forward<decltype(e)>(e));
});

Here, `e` is an element of the input range.

However, this can lead to ambiguity when `e` can also be converted to `initializer_list`, as `vector::insert` has an overload of insert(const_iterator, initializer_list<T>).