## Abstract
This pull request implements LWG3715: `view_interface::empty` is
overconstrained. Here is an example similar to those described in the
report, which compiles with `-stdlib=libstdc++` but failed to compile
with `-stdlib=libc++`:
```cpp
// https://godbolt.org/z/EWEoTzah3
std::istringstream input("1 2 3 4 5");
auto i = std::views::istream<int>(input);
auto r = std::views::counted(i.begin(), 4) | std::views::take(2);
assert(!r.empty());
```
## Reference
- [Draft C++ Standard:
[view.interface.general]](https://eel.is/c++draft/view.interface.general)
- [LWG3715](https://wg21.link/LWG3715)
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