clang++-libc++ on Trusty doesn't seem to support thread_local

[pdimov]

I added a few more configurations to Travis here: 0ac815e

and the clang++-libc++ one (which uses the provided libc++-dev package) fails when thread_local is used with

/home/travis/build/boostorg/boost-root/libs/config/test/boost_no_cxx11_thread_local.ipp:20: undefined reference to `__cxa_thread_atexit'

https://travis-ci.org/boostorg/config/jobs/329979139#L2474

I'm not quite sure what we ought to do about that; either define BOOST_NO_CXX11_THREAD_LOCAL for libc++ on Linux, or somehow figure out a way to make that config work. I think I tend towards the former.

(Having a working libc++ Linux Travis config is nice as it doesn't need to wait for the permanently clogged Mac jobs.)

[jzmaddock]

I get the impression that libc++ on Linux is somewhat under-supported. I've seen that error before - but can't remember where for now - and without using thread_local, just std lib code if I remember correctly.

I believe the fix is to link to libcxxabi - but I've never found it that easy to set up on Linux - is there a package?

[pdimov]

In this case everything else seems to work correctly, it's just thread_local that fails (and then presumably only in the case of a nontrivial destructor.)

smart_ptr tests pass on this config: https://travis-ci.org/boostorg/smart_ptr/jobs/330025116
and system passes too: https://travis-ci.org/boostorg/system/jobs/330093113

So it's not like the whole ABI is missing; just __cxa_thread_atexit. Presumably libsup++ gets linked in, why does its __cxa_thread_exit not work, no idea. The libc++-dev package contains no libc++-abi.so, just libc++.so, so that's not being used.

The command clang++-libc++ is just a shell script that calls clang++ -stdlib=libc++ and nothing else; and clang++ itself does support thread_local so it's somewhat of a mystery why -stdlib=libc++ breaks it.

No idea about libcxxabi. I generally try to keep the Travis configs as standard (as in, close to the system-provided) as possible, to mimic what real-world users would encounter.

All in all, it seems to me that defining BOOST_NO_CXX11_THREAD_LOCAL for Linux libc++ seems our best course at the moment.

I haven't tried g++-libc++ but no sane person would use that. :-)

[jzmaddock]

OK lets go with that for now and see how it goes.

[pdimov]

OK: #210