<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/91227>91227</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            Webassembly doesn't look for libc++ relative to clang, only under sysroot
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang,
            libc++
      </td>
    </tr>

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

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

<pre>
    ToolChains/WebAssembly.cpp addLibCxxIncludePaths calls detectLibcxxVersion on computeSysRoot()/include. If there's a c++/v. directory there, it adds system include paths for sysroot/include/triple/cxx/v and sysroot/include/cxx/v

GenericGCC and things derived from it (I found this for Linux) does more complicated things. It looks in various places for a plausible include path and when it finds one (or sometimes two) it stops looking for alternatives.

First try getDriver().Dir/../include/triple/c++/v and getDriver().Dir/../include/c++/v
Then try sysroot/usr/local/include
Then try sysroot/usr/include

Webassembly uses `libpath / MultiArchTriple / cxx` directly, Generic_GCC checks whether the directory exists as well. That's a behaviour difference but probably not a significant one - changing to not looking in a directory that doesn't exist seems OK to me.

I'd like to use the same set of libc++ headers for webassembly as for other targets but the workaround of copying them doesn't do much harm.

Creating this issue to ask:

1. Should webassembly look for libc++ at clang/../include/cxx/?
2. Should it adopt the getTargetSubDirPath pattern of not appending non-existing dirs?
3. If next to clang and sysroot both exist, do we want to explictly ignore one of them 

The change is straightforward if we want it. Thanks
</pre>
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