Build Proxygen under Ubuntu 16.04 - c++

Question

I would like to build Facebook's Proxygen c++ http libraries out of github with Ubuntu 16.04. Here's the environment I set up along with the deps.sh command to install dependencies:
gcc --version
gcc (Ubuntu 5.4.0-6ubuntu1~16.04.1) 5.4.0 20160609
export CPPFLAGS="-std=c++14"
export CXXFLAGS="-std=c++14"
git clone git#github.com:facebook/proxygen.git
cd proxygen/proxygen && ./deps.sh
That got me most of the way through building its folly dependency, but I am getting an incomplete type error:
libtool: compile: g++ -DHAVE_CONFIG_H -I./.. -pthread -I/usr/include -std=c++14 -std=gnu++1y -std=c++14 -MT io/async/AsyncPipe.lo -MD -MP -MF io/async/.deps/AsyncPipe.Tpo -c io/async/AsyncPipe.cpp -fPIC -DPIC -o io/async/.libs/AsyncPipe.o
In file included from /usr/include/c++/5/bits/move.h:57:0,
from /usr/include/c++/5/bits/stl_pair.h:59,
from /usr/include/c++/5/utility:70,
from /usr/include/c++/5/algorithm:60,
from ./../folly/Conv.h:26,
from Conv.cpp:16:
/usr/include/c++/5/type_traits: In instantiation of ‘struct std::make_unsigned<__int128>’:
Conv.cpp:528:52: required from ‘folly::detail::ConversionResult<T> folly::detail::digits_to(const char*, const char*) [with Tgt = __int128]’
Conv.cpp:658:16: required from here
/usr/include/c++/5/type_traits:1757:62: error: invalid use of incomplete type ‘class std::__make_unsigned_selector<__int128, false, false>’
{ typedef typename __make_unsigned_selector<_Tp>::__type type; };
^
/usr/include/c++/5/type_traits:1721:11: note: declaration of ‘class std::__make_unsigned_selector<__int128, false, false>’
class __make_unsigned_selector;
^
/usr/include/c++/5/type_traits: In instantiation of ‘struct std::make_unsigned<__int128 unsigned>’:
Conv.cpp:528:52: required from ‘folly::detail::ConversionResult<T> folly::detail::digits_to(const char*, const char*) [with Tgt = __int128 unsigned]’
Conv.cpp:661:16: required from here
Has anyone tried or solved this? I'm not familiar with the code base yet. Tia.

Answer 1

TL;DR Proxygen needs GNU extensions; use -std=gnu++11 or -std=gnu++14
Why do you need to override C++ standard when building proxygen and its dependencies? Folly itself specifies -std=gnu++1y. If you remove
export CPPFLAGS="-std=c++14"
export CXXFLAGS="-std=c++14"
and try to build it, it almost will, the only change that I had to make to folly is to fix membarrier.
If you insist on using -std=c++14, then the problem actually is not in folly, it's in libstdc++ handling of GNU extensions, this simple line:
typedef std::make_unsigned<__int128>::type int128_type;
Will easily compile with -std=gnu++11 or -std=gnu++1y, but will fail with any of -std=c++11 or -std=c++14. And it's hard to tell, whether it is a bug (because the compiler provides __int128 type (and folly detects that in its configure script, BTW) but C++ library has problems with it) or a feature (because __int128 is an extension in the first place and one should use some GNU variant of standard to properly get it).