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

[Roger Leigh]

Note that the strategy for GCC packaging in Ubuntu (maybe also in
Debian) is the exact opposite of how RedHat handle GCC packaging for
RHEL with the Devtoolset stuff.

Ubuntu is always providing the latest version of libstdc++6 from the
newest GCC.  This is problematic when the older headers are subtly (or
not-so-subtly) incompatible with the shared library.

RedHat always uses the copy of libstdc++ provided by the default
compiler for the base system.  Newer GCC versions link against this
library, with newer functionality provided by static linking of the
newer libstdc++ for that GCC version.

Both strategies make compromises.  But right now, the default GCC
version in Ubuntu is actually broken as a result of the former strategy.
Perhaps the default libstdc++6 should be provided by the default GCC
version, with later versions adding supplementary functionality via a
static 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:
  New

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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  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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