[Bug libstdc++/90557] New: Incorrect std::filesystem::path::operator=(std::filesystem::path const&) in gcc 9.1.0

[arnaud02 at users dot sourceforge.net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90557

            Bug ID: 90557
           Summary: Incorrect
                    std::filesystem::path::operator=(std::filesystem::path
                    const&) in gcc 9.1.0
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arnaud02 at users dot sourceforge.net
  Target Milestone: ---

Using gcc 9.1.0, I observe some memory issues detected with valgrind when using
std::filesystem:path:
==23251== Conditional jump or move depends on uninitialised value(s)
==23251==    at 0x6CDD45: void std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*,
char*, std::forward_iterator_tag) (basic_string.tcc:211)
==23251==    by 0xA807D5: _M_construct_aux<char*> (basic_string.h:247)
==23251==    by 0xA807D5: _M_construct<char*> (basic_string.h:266)
==23251==    by 0xA807D5: basic_string (basic_string.h:451)
==23251==    by 0xA807D5: path (fs_path.h:175)
==23251==    by 0xA807D5: _Cmpt (fs_path.h:690)
==23251==    by 0xA807D5: _Construct<std::filesystem::__cxx11::path::_Cmpt,
std::filesystem::__cxx11::path::_Cmpt&> (stl_construct.h:75)
==23251==    by 0xA807D5: __uninit_copy<std::filesystem::__cxx11::path::_Cmpt*,
std::filesystem::__cxx11::path::_Cmpt*> (stl_uninitialized.h:83)
==23251==    by 0xA807D5:
uninitialized_copy<std::filesystem::__cxx11::path::_Cmpt*,
std::filesystem::__cxx11::path::_Cmpt*> (stl_uninitialized.h:134)
==23251==    by 0xA807D5:
__uninitialized_copy_n<std::filesystem::__cxx11::path::_Cmpt*, int,
std::filesystem::__cxx11::path::_Cmpt*> (stl_uninitialized.h:767)
==23251==    by 0xA807D5:
uninitialized_copy_n<std::filesystem::__cxx11::path::_Cmpt*, int,
std::filesystem::__cxx11::path::_Cmpt*> (stl_uninitialized.h:814)
==23251==    by 0xA807D5:
std::filesystem::__cxx11::path::_List::operator=(std::filesystem::__cxx11::path::_List
const&) (fs_path.cc:281)
==23251==    by 0xA80858:
std::filesystem::__cxx11::path::operator=(std::filesystem::__cxx11::path
const&) (fs_path.cc:451)

I was not able to extract a small reproducer. However, the defect comes from
fs_path.cc:281

              std::uninitialized_copy_n(to + oldsize, newsize - oldsize,
                                        from + oldsize);
should be:
              std::uninitialized_copy_n(from + oldsize, newsize - oldsize,
                                        to + oldsize);