Some notes in `LLVM_ENABLE_LIBCXX`

[ldionne]

Summary

Reference: https://bugs.llvm.org/show_bug.cgi?id=43105

I've found a bug when libc++ is the only STL on system and cmake will fail building llvm.
It looks like this is not a serious issue so no one is working on that. But I think it's better to mention it in doc.

This is not a fix but just a document enhancement.

[erichkeane]

I would MUCH prefer that the compiler checker actually use the right library. Also, this isn't completely correct, MacOS Clang doesn't have libstdc++ available at all, yet works fine.

[oraluben]

In D66800#1647021, @erichkeane wrote:

I would MUCH prefer that the compiler checker actually use the right library. Also, this isn't completely correct, MacOS Clang doesn't have libstdc++ available at all, yet works fine.

You are right, it only fails if clang's default STL is not libc++. And I agree that this is not the proper way of resolving the issue.

[ldionne]

If your system only has libc++ available but the compiler shipped with it doesn't work out of the box (i.e. without passing -stdlib=libc++), isn't your compiler broken? In other words, it seems to me that your system compiler should always work out of the box without having to pass -stdlib=XXX -- do we agree? If so, I think the bug should be reported to whoever vends the compiler on the platform where you encountered this issue.

[erichkeane]

In D66800#1647121, @ldionne wrote:

If your system only has libc++ available but the compiler shipped with it doesn't work out of the box (i.e. without passing -stdlib=libc++), isn't your compiler broken? In other words, it seems to me that your system compiler should always work out of the box without having to pass -stdlib=XXX -- do we agree? If so, I think the bug should be reported to whoever vends the compiler on the platform where you encountered this issue.

I agree, and that makes a ton of sense to me. If your compiler doesn't work as shipped, it is an issue with the configuration.

I was finally able to view the bug, and it appears to be an ubuntu 18 docker image, compiling with Clang. I'm not sure how @oraluben managed to configure that system in a way to cause that problem, but I would think this is a configuration issue.

[oraluben]

In D66800#1647121, @ldionne wrote:

If your system only has libc++ available but the compiler shipped with it doesn't work out of the box (i.e. without passing -stdlib=libc++), isn't your compiler broken? In other words, it seems to me that your system compiler should always work out of the box without having to pass -stdlib=XXX -- do we agree? If so, I think the bug should be reported to whoever vends the compiler on the platform where you encountered this issue.

If I don't misunderstand, clang is supposed to use libc++ on Linux if there's not a libstdc++ installed?

[ldionne]

In D66800#1647210, @oraluben wrote:

In D66800#1647121, @ldionne wrote:

If your system only has libc++ available but the compiler shipped with it doesn't work out of the box (i.e. without passing -stdlib=libc++), isn't your compiler broken? In other words, it seems to me that your system compiler should always work out of the box without having to pass -stdlib=XXX -- do we agree? If so, I think the bug should be reported to whoever vends the compiler on the platform where you encountered this issue.

If I don't misunderstand, clang is supposed to use libc++ on Linux if there's not a libstdc++ installed?

Whatever the answer to that is (and I would guess it's "yes" but that's up to whoever produces the toolchain for Linux), that probably depends where libc++ is installed. Is your libc++ installed at "standard" locations on your Linux system?

[ldionne]

This has been a while and I still don't understand the purpose of this added note. Should we abandon this?

[ldionne]

Requesting changes so it shows up properly in the review queue.

[ldionne]

No answer in a while -- it looks like this isn't relevant anymore. Cleaning up the review queue.

llvm/docs/CMake.rst

**LLVM_USE_LINKER**:STRING
   search. For example to link LLVM with the Gold linker, cmake can be invoked
   with ``-DLLVM_USE_LINKER=gold``.
 
 **LLVM_ENABLE_LIBCXX**:BOOL
   If the host compiler and linker supports the stdlib flag, -stdlib=libc++ is
   passed to invocations of both so that the project is built using libc++
   instead of stdlibc++. Defaults to OFF.
 
+  .. note:: LLVM_ENABLE_LIBCXX will not affect cmake checking CXX compiler, so it will fail when `libc++` is the only STL on system. In that case `-DCMAKE_CXX_FLAGS='-stdlib=libc++'` should be used.
+
 **LLVM_STATIC_LINK_CXX_STDLIB**:BOOL
   Statically link to the C++ standard library if possible. This uses the flag
   "-static-libstdc++", but a Clang host compiler will statically link to libc++
   if used in conjuction with the **LLVM_ENABLE_LIBCXX** flag. Defaults to OFF.
 
 **LLVM_ENABLE_LLD**:BOOL
   This option is equivalent to `-DLLVM_USE_LINKER=lld`, except during a 2-stage
   build where a dependency is added from the first stage to the second ensuring