<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/123153>123153</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang does not warn about missing returns when a switch statement covers all enum values
        </td>
    </tr>

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

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

    <tr>
      <th>Reporter</th>
      <td>
          jeremy-rifkin
      </td>
    </tr>
</table>

<pre>
    The following code triggers a missing return warning by gcc, msvc, and edg but not clang:
```cpp
enum class E { a, b };

int foo(E e) {
    switch(e) {
        case E::a:
            return 20;
        case E::b:
 return 30;
    }
}
```
https://godbolt.org/z/b5r34nWf6

The main issue here is safety: This code is bug-prone due to extraneous enum values. The most common situation this could lead to bugs is for flag enums, e.g. https://godbolt.org/z/zx6nnPYMq.

Secondarily: This can lead to some annoyances where code compiles fine on clang but has a flood of warnings on gcc (possibly fatal ones, if using -Werror=return-type). This leads to PRs such as https://github.com/llvm/llvm-project/pull/122382 and https://github.com/llvm/llvm-project/pull/105520.

I'd like to propose changing the behavior to warn on this case by default. I briefly discussed this with folks on the llvm discord as well: https://discord.com/channels/636084430946959380/636732781086638081/1329124923322531850.
</pre>
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