Bug#763369: g++-4.9: Linker failure when using str::thread

[Ralf Jung <post@ralfj.de>]

Package: g++-4.9
Version: 4.9.1-15
Severity: normal

Dear Maintainer,

when compiling a C++11 program that uses std::thread, I get the following
linker failure:

$ g++ -std=c++11 threadtest.cpp -o threadtest
/tmp/cc2mc1l4.o: In function `std::thread::thread<void (*)()>(void (*&&)())':
threadtest.cpp:(.text._ZNSt6threadC2IPFvvEIEEEOT_DpOT0_[_ZNSt6threadC5IPFvvEIEEEOT_DpOT0_]+0x1e): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status

The program used to compile and run just fine around a month ago.
The problem also affects clang++. I will attach a small example program.

The program compiles and runs fine when I add "-pthread" to the flags
(this work-around applies to both g++ and clang++). However, it was
my understanding (and expectation) that this flag is no longer needed
with C++11 - in fact, I shouldn't even know that it's pthreads
implementing the std::thread class.

Kind regards
Ralf

-- System Information:
Debian Release: jessie/sid
  APT prefers testing
  APT policy: (990, 'testing'), (100, 'unstable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 3.15.10 (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages g++-4.9 depends on:
ii  gcc-4.9            4.9.1-15
ii  gcc-4.9-base       4.9.1-15
ii  libc6              2.19-11
ii  libcloog-isl4      0.18.2-1
ii  libgmp10           2:6.0.0+dfsg-6
ii  libisl10           0.12.2-2
ii  libmpc3            1.0.2-1
ii  libmpfr4           3.1.2-1
ii  libstdc++-4.9-dev  4.9.1-15
ii  zlib1g             1:1.2.8.dfsg-2

g++-4.9 recommends no packages.

Versions of packages g++-4.9 suggests:
pn  g++-4.9-multilib    <none>
ii  gcc-4.9-doc         4.9.1-2
pn  libstdc++6-4.9-dbg  <none>

-- no debconf information

#include <thread>
#include <iostream>

void thread_fun(void)
{
    std::cout << "In Thread" << std::endl;
}

int main(void)
{
    std::thread(&thread_fun).join();
    return 0;
}