Fix QUrl::fromLocalFile with long path prefix

After commit 3966b571 the function was kinda broken already, though
this got unnoticed since it was not covered by an the auto-test.
This commit adds another test case with Windows native separators
and removes the use of QDir::fromNativeSeparators. Instead use the
original code from QDir::fromNativeSeparators to replace the backslashes.

Pick-to: 5.15 6.0 6.1
Change-Id: I190560d0e75cb8c177d63b142aa4be5b01498da2
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/io/qurl.cpp

@@ -413,7 +413,6 @@
 #include "qstringlist.h"
 #include "qdebug.h"
 #include "qhash.h"
-#include "qdir.h"         // for QDir::fromNativeSeparators
 #include "qdatastream.h"
 #include "private/qipaddress_p.h"
 #include "qurlquery.h"
@@ -3291,6 +3290,25 @@ bool QUrl::isDetached() const
     return !d || d->ref.loadRelaxed() == 1;
 }
 
+static QString fromNativeSeparators(const QString &pathName)
+{
+#if defined(Q_OS_WIN)
+    QString result(pathName);
+    const QChar nativeSeparator = u'\\';
+    auto i = result.indexOf(nativeSeparator);
+    if (i != -1) {
+        QChar * const data = result.data();
+        const auto length = result.length();
+        for (; i < length; ++i) {
+            if (data[i] == nativeSeparator)
+                data[i] = u'/';
+        }
+    }
+    return result;
+#else
+    return pathName;
+#endif
+}
 
 /*!
     Returns a QUrl representation of \a localFile, interpreted as a local
@@ -3329,7 +3347,7 @@ QUrl QUrl::fromLocalFile(const QString &localFile)
     if (localFile.isEmpty())
         return url;
     QString scheme = fileScheme();
-    QString deslashified = QDir::fromNativeSeparators(localFile);
+    QString deslashified = fromNativeSeparators(localFile);
 
     // magic for drives on windows
     if (deslashified.length() > 1 && deslashified.at(1) == QLatin1Char(':') && deslashified.at(0) != QLatin1Char('/')) {

tests/auto/corelib/io/qurl/tst_qurl.cpp

@@ -1333,6 +1333,10 @@ void tst_QUrl::fromLocalFile_data()
                 << QString(QString("//somehost") + suffix).replace('/', '\\')
                 << QString("file://somehost") + suffix
                 << QString(suffix);
+        QTest::addRow("windows-backslash-extlen-%s", pathDescription)
+                << QString(QString("//?") + suffix).replace('/', '\\')
+                << QString("file:////%3F") + suffix
+                << QString("//?") + suffix;
 #endif
         QTest::addRow("windows-extlen-%s", pathDescription)
                 << QString("//?") + suffix