[cfe-dev] Inconsistent libc++ search paths

[Ilya Biryukov via cfe-dev]

Definitely a bug, not sure what should be the right fix (match includes
with libraries, the other way around or something in between), but wanted
to point out another oddity in described driver behavior.
We should not use -resource-dir for anything other than the internal clang
headers, tools like clang-tidy or clangd rely on this to override the
internal headers without overriding the C++ standard library.

On Sat, Feb 23, 2019 at 1:01 AM Dan Albert via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Maybe the reason they differ is so you get new libc++ headers but still
> use the system dylib by default on Mac? I know that's a use case that's
> fairly carefully maintained, but idk if it's intended to be the default.
>
I don't think that's the case. The search paths for libc++ on MacOS are
handled in "lib/Driver/ToolChains/Darwin.cpp" in
DarwinClang::AddClangCXXStdlibIncludeArgs.

>From the NDK side of things, changing the libc++ header search to match the
> libc++ library search should be fine.
>
> On Fri, Feb 22, 2019 at 3:20 PM Evgenii Stepanov <
> eugeni.stepanov at gmail.com> wrote:
>
>> Hi,
>>
>> in lib/Driver/ToolChains/Linux.cpp, the search order for libc++
>> headers and libraries seems inconsistent.
>>
>> Headers are searched in
>> 1. Resource dir
>> 2. Driver dir
>> 3. %sysroot/usr/include/c++
>>
>> Libraries are searched in
>> 1. Driver dir only if it is under sysroot (!)
>> 2. %sysroot/lib
>> 3. %sysroot/usr/lib
>>
>> When building with what Linux::addLibCxxIncludePaths calls
>> "development, non-installed" clang and a sysroot (ex. I'm using one
>> from Android NDK) where both locations have a copy of libc++, clang
>> picks libc++ headers from its resource dir, and an incompatible
>> library from sysroot.
>>
>> Comments around the library search code suggest that the order should
>> not be changed. Does it make sense to tweak the headers search order
>> to match? That way building with local libc++ and a sysroot would
>> require extra flags (because clang would default to sysroot headers),
>> but that is already the case for the -shared-libc++ case.
>>
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-- 
Regards,
Ilya Biryukov
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