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4239. flat_map's transparent comparator no longer works for string literals

Section: 23.6.8.7 [flat.map.modifiers] Status: New Submitter: Hui Xie Opened: 2025-03-29 Last modified: 2025-03-29

Priority: Not Prioritized

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Discussion:

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 23.6.8.7 [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.

Proposed resolution:

This wording is relative to N5008.

1. Modify 23.6.8.7 [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)...);