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

[Matthias Klose]

there is a link error with:

$ g++-8 -static-libstdc++ -std=c++17 -lstdc++fs -g3 -o testpath testpath.cpp
/usr/bin/ld: /tmp/ccWrH4Ho.o: in function `std::filesystem::__cxx11::path::path<char [5], std::filesystem::__cxx11::path>(char const (&) [5], std::filesystem::__cxx11::path::format)':
/usr/include/c++/8/bits/fs_path.h:184: undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
collect2: error: ld returned 1 exit status

you won't see it with dynamic linking because in 9, the code from
libstdc++fs is now integrated in the libstdc++ library.

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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
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  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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