A name rebuilt part by part loses its volume (lexically_normal, weakly_canonical, / with a rooted name) #68

Observation

With 0.19.1 (X:/ is a root name, #64), a name that libc++ rebuilds part by part loses its volume:

path("D:/a/./b/../c").lexically_normal()  ->  "/a/c"
path("D:/").lexically_normal()            ->  "/"
path("D:/a") / "/b"                       ->  "/b"

X: is a root name only with the separator after it, so lexically_normal, which rebuilds its result with /=, appends D: as an ordinary name and the root directory / then replaces it under the POSIX append rule. weakly_canonical ends in lexically_normal and loses the volume the same way.

On Windows a program over the hosted arrangement then names its files beneath no volume. mcpp's Windows launcher (mcpplibs.os process::exec, which normalizes the program's name) fails with cannot start Z:/…/okrun.exe under openkal-platform 0.24.1 (mcpp-community/mcpp#889, e2e 988); it starts under 0.24.0.

Expected

The result begins with the volume and its root directory, and appending a name with a root directory and no root name beneath a volume keeps the volume (D:/a / /b is D:/b), as [fs.path.append] states it for a system with root names.