<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/67338>67338</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `<ranges>`: range adaptor object unconditionally requires bound arguments to be move-constructible
        </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>
    Although the following is extremely contrived, sorry, it seems that it should be well-formed according to the current wording:

https://godbolt.org/z/EE647jeYn

```cpp
#include <ranges>

struct OnlyCopyable {
  OnlyCopyable() = default;
 OnlyCopyable(const OnlyCopyable&) = default;
 OnlyCopyable(OnlyCopyable&&) = delete;
  constexpr operator int() const { return 2; }
};

int main() {
  constexpr OnlyCopyable arg;
 // hard error in libc++ as std::bind_back constraint arguments to be move-constructible
  constexpr auto closure = std::views::take(arg);
 static_assert(std::string_view("ab") == closure(std::string_view("abcde")));
}
```

The issue here is that libc++ uses `std::__bind_back` to implement the `adaptor(args...)`, which is a constrained function that additionally requires `(is_move_constructible_v<std::decay_t<Args>> && ...)` compared to the wording in `<ranges>`.

It is worth noting that there is no isse with `adaptor | adaptor` as `std::__compose` is not a constraint function.

Not sure if this is worth filing an LWG.
</pre>
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