<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87221>87221</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [Compatibility] Option -w can't disable the warning -Wimplicit-function-declaration 
        </td>
    </tr>

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

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

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

<pre>
    * test: https://gcc.godbolt.org/z/b6djWGGW9
```
int square(int num) {
 memset (&num, 0, 4);
    return 0;
}
```

* since clang15, miss head file <string.h>for **memset** will report an **error**, so it can't be disable by add option `-w`, so many old code should modify the source to avoid this issue. Does we still have an option to compatible with old code ?
</pre>
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