[Bug libstdc++/94063] filesystem::path concatenation doesn't work for Windows root-paths

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94063

--- Comment #6 from Martin Liška <marxin at gcc dot gnu.org> ---
commit r9-8369-g7ef07b622d8c2fca35813bf50669dcd663fe5cf2
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Mar 12 17:39:05 2020 +0000

    libstdc++: Handle type-changing path concatenations (PR 94063)

    The filesystem::path::operator+= and filesystem::path::concat functions
    operate directly on the native format of the path and so can cause a
    path to mutate to a completely different type.

    For Windows combining a filename "x" with a filename ":" produces a
    root-name "x:". Similarly, a Cygwin root-directory "/" combined with a
    root-directory and filename "/x" produces a root-name "//x".

    Before this patch the implemenation didn't support those kind of
    mutations, assuming that concatenating two filenames would always
    produce a filename and concatenating with a root-dir would still have a
    root-dir.

    This patch fixes it simply by checking for the problem cases and
    creating a new path by re-parsing the result of the string
    concatenation. This is slightly suboptimal because the argument has
    already been parsed if it's a path, but more importantly it doesn't
    reuse any excess capacity that the path object being modified might
    already have allocated.

    Backport from mainline
    2020-03-09  Jonathan Wakely  <jwakely@redhat.com>

            PR libstdc++/94063
            * src/c++17/fs_path.cc (path::operator+=(const path&)): Add kluge
to
            handle concatenations that change the type of the first component.
            (path::operator+=(basic_string_view<value_type>)): Likewise.
            * testsuite/27_io/filesystem/path/concat/94063.cc: New test.