Proposed resolution:

This wording is relative to N4964.

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

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

   -12- Effects: Adds elements to c as if by:

   for (value_typeconst auto& e : rg) {
     c.keys.insert(c.keys.end(), std::move(e.first));
     c.values.insert(c.values.end(), std::move(e.second));
   }
   

   […]

[ 2024-02-22; Reflector poll ]

Set priority to 3 after reflector poll in October 2023.

"This is P2767 section 6 which LWG looked at in Varna, and was turned into https://github.com/cplusplus/draft/pull/6274 as an editorial change."

flat_map::insert_range adds elements to the containers member via:

for (const auto& e : rg) {
  c.keys.insert(c.keys.end(), e.first);
  c.values.insert(c.values.end(), e.second);
}

which is incorrect because rg's value type may not be a pair (tuple, for instance), which means that .first and .second are not valid in such cases.