with master as of 9fc16dbc

abidw --abidiff /home/ben/Shared/Work/spack/opt/spack/linux-fedora33-skylake/gcc-10.2.1/miniconda3-4.8.2-oudnbhbtypfh2ozp37mvcmbef4p2gjcv/lib/libstdc++.so.6.0.26

reports hundreds of differences with itself beginning with:

Functions changes summary: 0 Removed, 634 Changed, 0 Added functions
Variables changes summary: 0 Removed, 0 Changed, 0 Added variable

634 functions with some indirect sub-type change:

  [C] 'method void std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >::basic_istringstream(std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >&&)' at sstream:477:1 has some indirect sub-type changes:
    implicit parameter 0 of type 'std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >*' has sub-type changes:
      in pointed to type 'class std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >' at sstream:392:1:
        type size hasn't changed
        1 base class change:
          'class std::basic_istream<char, std::char_traits<char> >' at istream:58:1 changed:
            type size hasn't changed
            2 member function insertions:
              'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream()' at istream:103:1, virtual at voffset 0/0
              'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream(int, void**)' at istream:103:1
            2 member function changes:
              'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream(int, void**)' has some sub-type changes:
                implicit parameter 0 of type 'std::basic_istream<char, std::char_traits<char> >*' has sub-type changes:
                  pointed to type 'class std::basic_istream<char, std::char_traits<char> >' changed, as being reported
              'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream()' has some sub-type changes:
                implicit parameter 0 of type 'std::basic_istream<char, std::char_traits<char> >*' has sub-type changes:
                  pointed to type 'class std::basic_istream<char, std::char_traits<char> >' changed, as being reported
        2 member function changes:
          'method virtual std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >::~basic_istringstream(int, void**)' has some sub-type changes:
            implicit parameter 0 of type 'std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >*' has sub-type changes:
              pointed to type 'class std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >' changed, as being reported
          'method virtual std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >::~basic_istringstream()' has some sub-type changes:
            implicit parameter 0 of type 'std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >*' has sub-type changes:
              pointed to type 'class std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >' changed, as being reported
    parameter 1 of type 'std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >&&' has sub-type changes:
      referenced type 'class std::__cxx11::basic_istringstream<char, std::char_traits<char>, std::allocator<char> >' changed at sstream:392:1, as reported earlier

The origin appears to be:

 'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream()' at istream:103:1, virtual at voffset 0/0
 'method virtual std::basic_istream<char, std::char_traits<char> >::~basic_istream(int, void**)' at istream:103:1