See the discussion in https://reviews.llvm.org/rG1a92de00 for more context.

@rnk Are you able to provide a reproducer we could add to the test suite?

Thanks, I think this will solve the issue.

It's also worth noting that there is one _LIBCPP_ASSERT in locale:
https://github.com/llvm/llvm-project/blob/b894a9fb237345db64d14ce3881d3195e124df0d/libcxx/include/locale#L4017
But, missing that assert seems preferable to locale registration failure.

Here is the reproducer I came up with. I made a shared library and exe from these two cpp files and linked them with the shell script:

$ cat facet_lib.cpp 
#include <sstream>
std::string __attribute__((visibility("default"))) numToString(int x);
std::string numToString(int x) {
  std::stringstream ss;
  ss << x;
  return ss.str();
}

$ cat facet.cpp 
#include <string>
#include <cstdio>
std::string __attribute__((visibility("default"))) numToString(int x);
int main() {
  std::string s = numToString(42);
  printf("should be 42: %s\n", s.c_str());
}

$ cat linkit.sh 
#!/bin/bash
set -e
clang++ -g -D_LIBCPP_DEBUG=1 -fvisibility=hidden -stdlib=libc++ -c facet_lib.cpp -fPIC -o facet_lib.o
clang++ -stdlib=libc++ -fPIC facet_lib.o  -shared -o libfacet_lib.so
clang++ -g -D_LIBCPP_DEBUG=1 -fvisibility=hidden -stdlib=libc++ facet.cpp libfacet_lib.so -o facet
LD_LIBRARY_PATH=.:lib ./facet

$ ./linkit.sh 
should be 42: 
# output is wrong

objdump can confirm that there is a hidden ::id global in the facet_lib.o, which I think is the issue (double facet registration):

$ llvm-objdump -t facet_lib.o | c++filt | grep num_put.*::id
0000000000000000  w    O .bss._ZNSt3__17num_putIcNS_19ostreambuf_iteratorIcNS_11char_traitsIcEEEEE2idE  0000000000000010 .hidden std::__1::num_put<char, std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> > >::id

After I apply this patch locally and relink, when I check the object file, I see that the id static data member is not defined in this object:

$ llvm-objdump -t facet_lib.o | c++filt | grep num_put.*::id
0000000000000000         *UND*  0000000000000000 std::__1::num_put<char, std::__1::ostreambuf_iterator<char, std::__1::char_traits<char> > >::id

It is provided by the libc++ library instead, which is correct.

Hi @ldionne,

here is a problem with extern-templates.sh.cpp test on the windows-to-linux ARM cross builders

http://lab.llvm.org:8011/#/builders/60/builds/1666
http://lab.llvm.org:8011/#/builders/119/builds/1962

the problem looks like

/tmp/libcxx.3WqvVHTiFB/t.tmp.exe: error while loading shared libraries: C:/buildbot/as-builder-2/x-aarch64/build/runtimes/runtimes-bins/libcxx/test/libcxx/debug/Output/extern-templates.sh.cpp.dir/t.tmp.lib: cannot open shared object file: No such file or directory

clang stores a full path to t.tmp.lib into the test executable and this path is unavailable when the test gets running on the remote host (over ssh).

A possible fix is linking the test within local folder and pass a short library path into the linker such as ./t.tmp.lib. I.e. replace the second RUN line with something like

RUN: cd %T && %{cxx} %{flags} %{compile_flags} %s ./t.tmp.lib %{link_flags} -fPIC -DTU2 -D_LIBCPP_DEBUG=1 -fvisibility=hidden -o %t.exe

I have tested these changes on my local build system and it works, but I'm not sure how it will work on the other systems.

In D94718#2521489, @vvereschaka wrote:

Hi @ldionne,

[...]

A possible fix is linking the test within local folder and pass a short library path into the linker such as ./t.tmp.lib. I.e. replace the second RUN line with something like

RUN: cd %T && %{cxx} %{flags} %{compile_flags} %s ./t.tmp.lib %{link_flags} -fPIC -DTU2 -D_LIBCPP_DEBUG=1 -fvisibility=hidden -o %t.exe

I have tested these changes on my local build system and it works, but I'm not sure how it will work on the other systems.

Thanks for the detailed explanation. This should be fixed by 90407b16b1d3e38f1360b6a24ceab801ab9cefc1.