<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/61931>61931</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Error combining c++20, libc++, PCH and stdarg
        </td>
    </tr>

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

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

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

<pre>
    The following fails with clang-15 and clang-16 on ubuntu 18 (binaries from https://github.com/llvm/llvm-project/releases)

`pch.h`
```
#include <stdlib.h>
```

`pch.cpp`
```
#include "pch.h"
```

`test.cpp`
```
#include "pch.h"
#include <stdarg.h>
```

## Run
```
clang++ -stdlib=libc++ -std=c++20 -Xclang -emit-pch -o pch.pch -c pch.cpp
clang++ -stdlib=libc++ -std=c++20 -Xclang -include-pch -Xclang pch.pch -c test.cpp
```


## Error
```
In file included from test.cpp:2:
/opt/clang-16/lib/clang/16/include/stdarg.h:14:9: error: reference to '__builtin_va_list' is ambiguous
typedef __builtin_va_list __gnuc_va_list;
        ^
note: candidate found by name lookup is '__builtin_va_list'
note: candidate found by name lookup is '__builtin_va_list'
```

Building without PCHs (i.e. `clang++ -stdlib=libc++ -std=c++20 -c test.cpp`) works fine.

## Godbolt
This is also reproducible on godbolt: https://godbolt.org/z/xTen4ae65
</pre>
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