[Bug 1824721] Re: g++-8 in disco is broken with libstdc++6 from gcc9 and libstdc++fs

[Jonathan Wakely]

The GCC 8 std::filesystem (and std::experimental::filesystem) symbols
are in libstdc++fs.a which is only provided as a static library, so link
order does matter. If you don't tell the linker to use it after the
objects that require it, then no symbols from libstdc++fs.a will be
used.

In GCC 9 the std::experimental::filesystem symbols are still in
libstdc++fs.a but the std::filesystem symbols are in libstdc++.so.6.0.26
now, but they're incompatible with the GCC 8 definitions. In your
problematic link no symbols were used from libstdc++fs.a, so the linker
finds the GCC 9 ones in libstdc++.so.6.0.25 and uses them, but that
leads to crashes.

It would have been possible to make the GCC 8 and 9 std::filesystem
symbols mangle differently, and maybe I should do that for GCC 8.4, but
for now the short answer is "C++17 support in GCC 8 is experimental, the
onus is on you to link correctly".

Since this problem is specific to Ubuntu, due to mixing gcc-8 and
libstdc++.so.6.0.26, another possibility would be for Ubuntu's gcc-8 to
reorder the linker arguments. If -lstdc++fs is given it could be moved
to after the object files (but before the implicit -lstdc++ option that
g++ adds).

Another option would be to make g++ simply link to -lstdc++fs
implicitly, as it does for -lstdc++. Unless the user adds -Wl,--whole-
archive (and doesn't follow it with -Wl,--no-whole-archive) that should
be harmless because the symbols in libstdc++fs.a will only be used if
actually needed.

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https://bugs.launchpad.net/bugs/1824721

Title:
  g++-8 in disco is broken with libstdc++6 from gcc9 and libstdc++fs

Status in gcc-8 package in Ubuntu:
  Incomplete

Bug description:
  There is a significant incompatibility here which leaves the default
  C++ compiler broken with C++17 std::filesystem usage.  Unfortunately,
  there isn't an obvious workaround since there's only one libstdc++6
  package, and it's using the non-default and unreleased GCC9 version of
  libstdc++, making it impossible to use the compatible GCC8 version of
  the library.

  % cat testpath.cpp
  #include <filesystem>

  int main()
  {
    std::filesystem::path p("test");
  }

  g++-9 -std=c++17 -lstdc++fs -g3 -o testpath testpath.cpp 
  % ./testpath

  % g++-8 -std=c++17 -lstdc++fs -g3 -o testpath testpath.cpp
  % ./testpath                                              
  zsh: segmentation fault (core dumped)  ./testpath

  % gdb testpath
  GNU gdb (Ubuntu 8.2.91.20190405-0ubuntu3) 8.2.91.20190405-git
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  For help, type "help".
  Type "apropos word" to search for commands related to "word"...
  Reading symbols from testpath...
  (gdb) run
  Starting program: /tmp/testpath 

  Program received signal SIGSEGV, Segmentation fault.
  0x000055555555649b in std::vector<std::filesystem::__cxx11::path::_Cmpt, std::allocator<std::filesystem::__cxx11::path::_Cmpt> >::~vector (this=0x23, __in_chrg=<optimised out>) at /usr/include/c++/8/bits/stl_vector.h:567
  567             std::_Destroy(this->_M_impl._M_start, this->_M_impl._M_finish,
  (gdb) bt
  #0  0x000055555555649b in std::vector<std::filesystem::__cxx11::path::_Cmpt, std::allocator<std::filesystem::__cxx11::path::_Cmpt> >::~vector (this=0x23, __in_chrg=<optimised out>) at /usr/include/c++/8/bits/stl_vector.h:567
  #1  0x0000555555556314 in std::filesystem::__cxx11::path::~path (this=0x3, __in_chrg=<optimised out>)
      at /usr/include/c++/8/bits/fs_path.h:208
  #2  0x0000555555556f5c in std::filesystem::__cxx11::path::_Cmpt::~_Cmpt (this=0x3, __in_chrg=<optimised out>)
      at /usr/include/c++/8/bits/fs_path.h:643
  #3  0x0000555555556f77 in std::_Destroy<std::filesystem::__cxx11::path::_Cmpt> (__pointer=0x3)
      at /usr/include/c++/8/bits/stl_construct.h:98
  #4  0x0000555555556e27 in std::_Destroy_aux<false>::__destroy<std::filesystem::__cxx11::path::_Cmpt*> (
      __first=0x3, __last=0x0) at /usr/include/c++/8/bits/stl_construct.h:108
  #5  0x0000555555556a98 in std::_Destroy<std::filesystem::__cxx11::path::_Cmpt*> (__first=0x3, __last=0x0)
      at /usr/include/c++/8/bits/stl_construct.h:137
  #6  0x00005555555567a9 in std::_Destroy<std::filesystem::__cxx11::path::_Cmpt*, std::filesystem::__cxx11::path::_Cmpt> (__first=0x3, __last=0x0) at /usr/include/c++/8/bits/stl_construct.h:206
  #7  0x00005555555564b1 in std::vector<std::filesystem::__cxx11::path::_Cmpt, std::allocator<std::filesystem::__cxx11::path::_Cmpt> >::~vector (this=0x7fffffffe310, __in_chrg=<optimised out>)
      at /usr/include/c++/8/bits/stl_vector.h:567
  #8  0x0000555555556314 in std::filesystem::__cxx11::path::~path (this=0x7fffffffe2f0, __in_chrg=<optimised out>)
      at /usr/include/c++/8/bits/fs_path.h:208
  #9  0x0000555555556230 in main () at testpath.cpp:5
  (gdb) quit
  A debugging session is active.

          Inferior 1 [process 2414] will be killed.

  Quit anyway? (y or n) y

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