<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/138526>138526</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [clang] Warning for comparing `char8_t` to `char32_t`
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

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

<pre>
    Consider the following code:
```cpp
bool contains_oe(std::u8string_view str) {
    for (char8_t c : str)
        if (c == U'ö') // comparison always fails
            return true;
    return false;
}
```
If `str` is a correctly encoded UTF-8 string, the comparison always fails because no UTF-8 code unit can be `0x6F`, and `ö` is U+00F6. Comparing `charN_t` with different `N` is virtually always a bug, or could have just as well been written using a different type of literal. Comparing these types is not going to give meaningful results except for U+007F and below, and even then, it's unclear why you wouldn't use the proper type.

I've floated the idea of deprecating this behavior in the C++ standard in a number of places, and it was received positively. StackOverflow users also suggested getting rid of it here: https://stackoverflow.com/q/79604433/5740428

**In the meantime, it would be useful to have a warning when `charN_t` is converted to a different Unicode character type.** This warning should be triggered for any implicit conversion, not just as part of a comparison because the same bug can be produced like:
```cpp
bool contains_char(std::u8string_view str, char8_t c);
// ...
contains_char(U'ö');
```
</pre>
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