<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/125863>125863</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            __datasizeof() does not ignore tail padding.
        </td>
    </tr>

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

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

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

<pre>
    ```C++
#include <cstdint>

struct Tst {
 std::uint32_t u1;
    std::uint8_t u2;
};


static_assert(sizeof(Tst) == 8);
static_assert(__datasizeof(Tst) == 5);
```

`__datasizeof(Tst)` returns 8 instead of 5.
If I'm missing something and it should work that way, than it would be good to clarify that here: https://clang.llvm.org/docs/LanguageExtensions.html#implementation-defined-keywords

https://godbolt.org/z/ejPYcs8oq
</pre>
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