<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/134347>134347</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `<flat_set>`: `flat_set::emplace()` is underconstrained
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          hewillk
      </td>
    </tr>
</table>

<pre>
    [[flat.set.modifiers]](https://eel.is/c++draft/flat.set.modifiers#1):

> ```cpp
> template<class K> constexpr pair<iterator, bool> insert(K&& x);
> template<class K> constexpr iterator insert(const_iterator hint, K&& x);
> ```
> Constraints: The _qualified-id_ `Compare​::​is_transparent` is valid and denotes a type. `is_constructible_v<value_type, K>` is `true`.

This means that the following code is well-formed since we will not enter the `emplace` branch:

```cpp
#include <flat_set>
#include <ranges>
#include <vector>

int main() {
  std::vector<int> v;
  auto m = std::ranges::to<std::flat_set<int>>(v, v);
}
```
https://godbolt.org/z/6hj55Mqdr

</pre>
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