[Bug c++/77285] New: extern thread_local linkage

["jan.willem.ps at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77285

            Bug ID: 77285
           Summary: extern thread_local linkage
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jan.willem.ps at gmail dot com
  Target Milestone: ---

Minimal test case:

cat > a.h <<EOF
#pragma once
#include <string>
extern thread_local std::string gFeelingLucky;
EOF

cat > a.cpp <<EOF
#include "a.h"
thread_local std::string gFeelingLucky = "Lucky";
EOF

cat > main.cpp <<EOF
#include "a.h"
#include <iostream>

int main() {
std::cout << "I'm feeling " << gFeelingLucky << '\n';
}
EOF

c++ -std=c++11 main.cpp a.cpp -o main
Output:
/tmp/cc1AJnUy.o: In function `_ZTW13gFeelingLucky':
main.cpp:(.text._ZTW13gFeelingLucky[_ZTW13gFeelingLucky]+0x5): undefined
reference to `_ZTH13gFeelingLucky'
collect2: error: ld returned 1 exit status

I've used the docker images on dockerhub to verify:
docker run -it --rm gcc:5.1 bash

This does work on gcc 4.9.0 (docker run --rm -it gcc:4.9.0 bash), so it looks
like a regression (see bug #55800).

It also doesn't work with gcc 6.1 (docker run -it --rm gcc:6.1 bash).