The example in [fs.path.decompose] p7 includes:

path("//host").filename();  // yields ""

This result isn't guaranteed, it depends whether the implementation interprets "//host" as a root-name or as a root-directory (with a redundant directory-separator) followed by the filename "host".

The example should make it clear that this interpretation is allowed.

Previous resolution [SUPERSEDED]:

This wording is relative to N4727.

1. Edit [fs.path.decompose] as indicated:

   path filename() const;
   

   -6- Returns: relative_path().empty() ? path() : *--end().

   [Example:

   path("/foo/bar.txt").filename(); // yields "bar.txt"
   path("/foo/bar").filename();     // yields "bar"
   path("/foo/bar/").filename();    // yields ""
   path("/").filename();            // yields ""
   path("//host").filename();       // yields "" or "host"
   path(".").filename();            // yields "."
   path("..").filename();           // yields ".."
   

   — end example]