QFileDialog: refactor a static helper

- Fix narrowing conversion warnings
- Don't use an out of bounds index with QStringView::mid(), which
  happened when view.size() was used as an index
- Use sliced()

Change-Id: Ia9bf62887ffb6ddd2458c9e46d33e8cfe0ee2b66
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/widgets/dialogs/qfiledialog.cpp

@@ -1134,20 +1134,28 @@ Q_AUTOTEST_EXPORT QString qt_tildeExpansion(const QString &path)
 {
     if (!path.startsWith(u'~'))
         return path;
-    int separatorPosition = path.indexOf(QDir::separator());
+
-    if (separatorPosition < 0)
+    if (path.size() == 1) // '~'
-        separatorPosition = path.size();
+        return QDir::homePath();
-    if (separatorPosition == 1) {
+
-        return QDir::homePath() + QStringView{path}.mid(1);
+    QStringView sv(path);
-    } else {
+    const qsizetype sepIndex = sv.indexOf(QDir::separator());
+    if (sepIndex == 1) // '~/' or '~/a/b/c'
+        return QDir::homePath() + sv.sliced(1);
+
 #if defined(Q_OS_VXWORKS) || defined(Q_OS_INTEGRITY)
-        const QString homePath = QDir::homePath();
+    if (sepIndex == -1)
+        return QDir::homePath();
+    return QDir::homePath() + sv.sliced(sepIndex);
 #else
-        const QByteArray userName = QStringView{path}.mid(1, separatorPosition - 1).toLocal8Bit();
+    const qsizetype userNameLen = sepIndex != -1 ? sepIndex - strlen("~") // '~user/a/b'
-        QString homePath = homeDirFromPasswdEntry(path, userName);
+                                                 : path.size() - strlen("~"); // '~user'
-#endif
+    const QByteArray userName = sv.sliced(1, userNameLen).toLocal8Bit();
-        return homePath + QStringView{path}.mid(separatorPosition);
+    QString homePath = homeDirFromPasswdEntry(path, userName);
-    }
+    if (sepIndex == -1)
+        return homePath;
+    return homePath + sv.sliced(sepIndex);
+#endif // defined(Q_OS_VXWORKS) || defined(Q_OS_INTEGRITY)
 }
 #endif