<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/118697>118697</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Clang mishandles GNU attributes on function declarations with trailing return type
        </td>
    </tr>

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

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

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

<pre>
    [Testcase demonstrating GCC behavior](https://godbolt.org/z/Trc91rY7d):

```c++
auto f() -> bool __attribute__((noreturn));
auto f() -> bool { return true; }

auto g() -> int __attribute__((vector_size(4)));
auto g() -> int { return true; }
```

GCC treats the `noreturn` attribute as applying to the function, and warns on the function definition because it does return.

GCC treats the `vector_size` attribute as applying to the return type, and rejects the definition because it specifies a different return type.

Clang treats the attribute as always applying to the return type. This seems like a missing case of GNU attribute "sliding".

FWIW, I encountered this specifically for the `diagnose_if` attribute, which can *only* be written as a GNU attribute, and because its argument references function parameters, can *only* be written in trailing position in a function declaration. There appears to currently be *no way* to combine trailing return types with the `diagnose_if` attribute (and other attributes like it, such as `enable_if` and the thread-safety annotations), which is a problem for coding styles / standards that mandate the use of trailing return types.

Presumably Clang should slide non-type attributes from a trailing return type onto the function declarator.
</pre>
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