<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/89388>89388</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `scoped_lock` constructor could be `nodiscard` like in `libstdc++`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            enhancement,
            libc++
      </td>
    </tr>

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

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

<pre>
    Consider this [code](https://godbolt.org/z/fror15jfd):
```c++
#include <mutex>

int num;
std::mutex m;
void top() {
    if (std::scoped_lock{m}; num > 10) { // no-warning :(
        num = 1;
 }
    if (std::lock_guard{m}; num > 11) { // warn: nodiscard
 num = 2;
    }
}
```

The constructor of `std::scoped_lock` should have the `[[nodiscard]]` attribute to catch mistakes like the one in the example, creating a temporary `scoped_lock` object and leaving the access of the global variable unguarded.

If we use libstdc++, we get the following warning `ignoring temporary created by a constructor declared with 'nodiscard' attribute [-Wunused-value]`. Maybe we could also have this `nodiscard` attribute too.

Note that for instance `lock_guard` works as expected, and has the ``nodiscard` attribute.
</pre>
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