This is brought up in the LWG reflector. We currently call `reserve` if
the underlying container has one. But the spec does not specify what
`reserve` should do for Sequence Container. So in theory if the
underlying container is user defined type and it can have a function
called `reserve` which does something completely different.
The fix is to just call `reserve` for STL containers if it has one
See discussion in https://cplusplus.github.io/LWG/issue4239
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
https://wg21.link/P2865R6.
This patch uses `std::upper_bound` instead.
Unlike `flat_map` and `flat_multimap`, The two function template
overloads `flat_set::insert`'s wording strongly suggest we should use
the transparent comparator
https://eel.is/c++draft/flat.set#modifiers-1
Both the code and the tests were not using the transparent comparator,
which needs to be fixed
