[Bug libstdc++/106127] New: experimental::filesystem::path("//") should be a root-directory not a root-name

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106127

            Bug ID: 106127
           Summary: experimental::filesystem::path("//") should be a
                    root-directory not a root-name
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

Our Filesystem TS implementation treats // the same as //x which is probably
wrong.

It causes experimental/filesystem/operations/absolute.cc to FAIL on Windows,
because absolute(path("//"), "z:\\tmp") returns "//\\tmp" which is a root-name.
(On the other hand, the specification for experimental::filesystem::absolute is
known to be broken for Windows, and was completely changed for C++17).