Proposed resolution:

This wording is relative to N5008.

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

   template<class K, class... Args>
     constexpr pair<iterator, bool> try_emplace(K&& k, Args&&... args);
   template<class K, class... Args>
     constexpr iterator try_emplace(const_iterator hint, K&& k, Args&&... args);
   

   -19- Constraints: […]

   -20- Preconditions: […]

   -21- Effects: If the map already contains an element whose key is equivalent to `k`, `*this` and `args...` are unchanged. Otherwise equivalent to:

   auto key_it = upper_bound(c.keys.begin(), c.keys.end(), k, compare)ranges::upper_bound(c.keys, k, compare);
   auto value_it = c.values.begin() + distance(c.keys.begin(), key_it);
   c.keys.emplace(key_it, std::forward<K>(k));
   c.values.emplace(value_it, std::forward<Args>(args)...);
   

According to the spec, the following code should hard error

std::flat_map<std::string, int, std::less<>> m;
m.try_emplace("abc", 5);  // hard error

The reason is that we specify in [flat.map.modifiers] p21 the effect to be as if `ranges::upper_bound` is called.

`ranges::upper_bound` requires `indirect_strict_weak_order`, which requires the comparator to be invocable for all combinations. In this case, it requires

const char (&)[4] < const char (&)[4] 

to be well-formed, which is no longer the case in C++26 after P2865R6.

We should just use `std::upper_bound` instead. libstdc++ already uses `std::upper_bound`. libc++ uses `ranges::upper_bound` but clang has not yet implemented P2865 properly.