QTextStream: Fix logical error in setEncoding

We only want to enable writing BOM if we have _not_ started
writing.

Fixes: QTBUG-106279
Change-Id: Ibcbc101b931615fddb2507f01307bf9619772d7b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit c76f7cbbcbe68bf785e8d8cbbe641cd6c0e42834)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 56 insertions, 1 deletions

diff --git a/src/corelib/serialization/qtextstream.cpp b/src/corelib/serialization/qtextstream.cpp
index 5b0ab7fb47..ca2f0684d4 100644
--- a/src/corelib/serialization/qtextstream.cpp
+++ b/src/corelib/serialization/qtextstream.cpp
@@ -2941,7 +2941,7 @@ void QTextStream::setEncoding(QStringConverter::Encoding encoding)
 
     d->encoding = encoding;
     d->toUtf16 = QStringDecoder(d->encoding);
-    bool generateBOM = d->hasWrittenData && d->generateBOM;
+    bool generateBOM = !d->hasWrittenData && d->generateBOM;
     d->fromUtf16 = QStringEncoder(d->encoding,
                                   generateBOM ? QStringEncoder::Flag::WriteBom : QStringEncoder::Flag::Default);
 
diff --git a/tests/auto/corelib/serialization/qtextstream/tst_qtextstream.cpp b/tests/auto/corelib/serialization/qtextstream/tst_qtextstream.cpp
index 6d80efe956..cf1049ab4f 100644
--- a/tests/auto/corelib/serialization/qtextstream/tst_qtextstream.cpp
+++ b/tests/auto/corelib/serialization/qtextstream/tst_qtextstream.cpp
@@ -12,6 +12,7 @@
 #include <QDebug>
 #include <QElapsedTimer>
 #include <QFile>
+#include <QTemporaryFile>
 #include <QStringConverter>
 #include <QTcpSocket>
 #include <QTemporaryDir>
@@ -202,6 +203,9 @@ private slots:
 
     void textModeOnEmptyRead();
 
+    void autodetectUnicode_data();
+    void autodetectUnicode();
+
 private:
     void generateLineData(bool for_QString);
     void generateAllData(bool for_QString);
@@ -3043,6 +3047,57 @@ void tst_QTextStream::textModeOnEmptyRead()
     QVERIFY(file.isTextModeEnabled());
 }
 
+void tst_QTextStream::autodetectUnicode_data()
+{
+    QTest::addColumn<QStringConverter::Encoding>("encoding");
+    QTest::newRow("Utf8") << QStringConverter::Utf8;
+    QTest::newRow("Utf16BE") << QStringConverter::Utf16BE;
+    QTest::newRow("Utf16LE") << QStringConverter::Utf16LE;
+    QTest::newRow("Utf32BE") << QStringConverter::Utf32BE;
+    QTest::newRow("Utf32LE") << QStringConverter::Utf32LE;
+}
+
+void tst_QTextStream::autodetectUnicode()
+{
+    QFETCH(QStringConverter::Encoding, encoding);
+
+    QTemporaryFile file;
+    QVERIFY(file.open());
+
+    QString original("HelloWorld👋");
+
+    {
+        QTextStream out(&file);
+        out.setGenerateByteOrderMark(true);
+        out.setEncoding(encoding);
+        out << original;
+    }
+    file.seek(0);
+    {
+        QTextStream in(&file);
+        QString actual;
+        in >> actual;
+        QCOMPARE(actual, original);
+        QCOMPARE(in.encoding(), encoding);
+    }
+    file.seek(0);
+    // Again, but change order of calls to QTextStream...
+    {
+        QTextStream out(&file);
+        out.setEncoding(encoding);
+        out.setGenerateByteOrderMark(true);
+        out << original;
+    }
+    file.seek(0);
+    {
+        QTextStream in(&file);
+        QString actual;
+        in >> actual;
+        QCOMPARE(actual, original);
+        QCOMPARE(in.encoding(), encoding);
+    }
+}
+
 
 // ------------------------------------------------------------------------------