• Create a directory containing the ä character and run the program below:

#include <QCoreApplication>
#include <QFileSystemWatcher>
#include <QDebug>
#include <QFile>

namespace {

const auto dir = "/YOUR_PATH/Täst";

class Watcher : public QFileSystemWatcher
{
public:
  Watcher()
  {
    connect(this, &QFileSystemWatcher::directoryChanged, this, [](const QString& dir) { qDebug()<<"dir changed:"<<dir; });
  }
};

}//< ns

int main(int argc, char *argv[])
{
  QCoreApplication a(argc, argv);

  Watcher w;
  qDebug()<<"add path success?"<<w.addPath(dir);

  return a.exec();
}

• Add/remove files into/from the created folder

• You will not see "dir changed:" printed on modification

Reason

The folder path is added to the watcher's internal hash with one encoding (using QString::fromLocal8Bit), but when a change occurs, the folder to query is with different encoding (using QFile::decodeName), thus the hash fails to find a match as if the folder is not monitored at all.

Workaround

None! Even if someone adds a string with an explicit QFile::decodeName encoding, the string is always first "converted" by a QString::fromLocal8Bit call.

Solution

The path added must be with the exact same encoding as the one that is queried later.

We must either fixup QFseventsFileSystemWatcherEngine::addPaths to explicitly store a QFile::decodeName path
or
Fix FileSystemEngine::canonicalName (used by QFileInfo::canonicalFilePath, used by QFseventsFileSystemWatcherEngine::addPaths) to use QFile::decodeName instead QString::fromLocal8Bit:

/qt5/qtbase/src/corelib/io/qfilesystemengine_unix.cpp
QFileSystemEngine::canonicalName
line: 289:

QString canonicalPath = QDir::cleanPath(/*QString::fromLocal8Bit*/QFile::decodeName(ret));

This works

Question: What is the difference b/w QString::fromLocal8Bit and QFile::decodeName? To what extend are interchangeable?