[REG 6.8.0 -> 6.8.1] QUrl::resolved() can produce an invalid path like "/../"

Since Qt 6.8.1, resolving a local-file URL can produce an invalid path like "/../", for which QFileInfo{url.path()}.isRelative() returns false. Adjusting such an invalid-path URL via url=url.adjusted(QUrl::NormalizePathSegments) does not change it either.

For example, with Qt 6.8.1, the following code

auto url = QUrl::fromUserInput(QStringLiteral("/"));
QUrl relative(QStringLiteral("../"));
auto resolved = url.resolved(relative);
qCritical() << url << relative << resolved;

prints
QUrl("file:///") QUrl("../") QUrl("file:///../")

With Qt 5.15.16 (and, as far as I know, with Qt 6.8.0), the same code prints
QUrl("file:///") QUrl("../") QUrl("file:///")

The same issue also affects the following values of relative:

• ".."
• "../../../"
• "../rel"

Something similar happens on Windows, but in fewer scenarios:

QUrl::fromUserInput("C:/").resolved("../../../") produces the path "/.." instead of "C:";
QUrl::fromUserInput("C:/").resolved("../../relative") produces the path "/relative" instead of "C:/relative";
QUrl::fromUserInput("C:/").resolved("../../../../../../../invalid") produces the path "/../../../../../invalid" instead of "C:/invalid";

(the Windows examples may contain mistakes, because I have inferred them from KDevelop CI test failures)

The behavior of older Qt versions matches standard operating system command line behaviors of the command cd .. :

• running cd .. in the root directory in Bash keeps the root directory current (no change);
• running cd .. in the root directory of a drive in Windows cmd keeps the drive root directory current (no change).

This regression causes failures of KDevelop's TestPath unit test, but I am not aware of any problems for non-test code.