<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/129746>129746</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            `-isystem` does not suppress warnings from macros
        </td>
    </tr>

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

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

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

<pre>
    Given a header `provoke_warning.hpp`:
```cpp
enum struct E { A, B, C };

namespace N {
 using enum E;
}

#define PROVOKE_WARNING using enum E;
```

And a source file `main.cpp`:
```cpp
#include <provoke_warning.hpp>

struct S {
  PROVOKE_WARNING;
};
```

When compile with:
`clang -std=c++17 -isystem . -c main.cpp`

We get:
```console
main.cpp:4:3: warning: using enum declaration is a C++20 extension [-Wc++20-extensions]
    4 | PROVOKE_WARNING;
      |   ^
./provoke_warning.hpp:7:31: note: expanded from macro 'PROVOKE_WARNING'
    7 | #define PROVOKE_WARNING using enum E;
 |                               ^ 
```

Notice that the warning we _would_ get from the `using enum` inside the namespace is not emitted (because of `-isystem`). However the use of the macro still gives a warning despite the fact that the macro came from a header included under `-isystem`.

--

I see why this could be difficult to remedy, and I already see the counterarguments: "it's a macro! It's in your code, not the library code!" So it's understandable why this happens, but that doesn't make it correct or expected.

This comes up in real code: https://github.com/catchorg/Catch2/issues/2910

</pre>
<img width="1" height="1" alt="" src="http://email.email.llvm.org/o/eJyUVUGPozgT_TWVSykRmCQkBw4k3Zmv9Uk9q5nV9rFl7AK8Y2xkm2Ty71cG0p3t7R1pURRblOvVe89lw71XjSEqYHOAzcOCD6G1riBdkbGLyspr8UWdySDHlrgkh7BNemfP9ge9XrgzyjSrtu9hm0BWQlLGyfgTfQ9JSWbo0Ac3iICPCPkBS2BHPMS_I0L-ANkhZiWl4R35ngvC57gOkhIHr0yDI8TjvC5_mFYDyyTVyhD-9u3rH1____j6Un57fnr-8lnSjdKUWhqJHL0dnCCslaYoqePKrMS_6wCWKSP0IAkhO35mQPY4wc9iv99EfCT4LuQzci8tGRS26yOviwrtGxuhuWlw6YOE7EEAOwA7pDkulb_6QB2ucCnwXsaER9hQ-CjJGm81QVK-rc_KNWRlBlmJs6g4vfNSktDc8aCsQeWR43GiwBKkn4GMjwHYHJYv4hZYvgU8bB5GLxDXCPnxc09wfGIYETbRzhWw06del3lkm0aOxgaKI_3suZEksXa2w44LZxFY_rESy-dK-Vjpv_TRTO1XD2we8Z-b-myDEoSh5QFDSzeH8UL4erGDlq9xkybiMQ7b5L06bBNUxitJY-z9mCgftSN1KgSSCGxXkeCDJ7R1hLh1RqTB9iv8n73QmdyIMq-K08kpH5TW2Kgzxb29EZTkexWmwjUX4V3ClCV4RxPtt-thPiUSBzPfFnc8VpMfy-U0PqEnwkt7xdAqjyJagRWhVHWtxKADBouOOpLXeF9wI_EJuXbE5XVMjUyEHUwgx10zdGSCj70AjKkALI9aRqbAUnya3iiDVzs4FFZSRI0eRhytKsfddX6fAmP43eIMM4rxgRvJK31HueV9T8ZHnGqY7ZGWvAGWB-z4D0IVUFjnSAS0LnYpiUByduL3SXdHHoc-UnPE9UQhK7ENoY96gJ2AnRoV2qFaCdsBOwkeRGtdA-x0jFMG7KS8H8gDO7F9GjtvIYtM7rM9X1CR5us0TXb7fLdoi2q33dUkeJZmNV-zfC9I7FNeVWkmeMrThSpYwjZJlqzTnLHNbkX5bl-l-3q3y3YbkgTrhDqu9Errc7eyrlmMxYuU7fP1dqF5RdqP3xTGDF1wjAJj8RPjipi0rIbGwzrRygf_DhNU0FT8vWtGR8d98kPfO_L-1qD-7rD7xeB08QvLYo15WPbO_kki3Js2Uz8X7K8AAAD__0TGMA0">