diff --git a/src/corelib/tools/qstringbuilder.cpp b/src/corelib/tools/qstringbuilder.cpp
index a044cca3c9..2fe556096b 100644
--- a/src/corelib/tools/qstringbuilder.cpp
+++ b/src/corelib/tools/qstringbuilder.cpp
@@ -108,12 +108,24 @@ void QAbstractConcatenable::convertFromAscii(const char *a, int len, QChar *&out
     if (len == -1) {
         if (!a)
             return;
-        while (*a)
+        while (*a && uchar(*a) < 0x80U)
             *out++ = QLatin1Char(*a++);
+        if (!*a)
+            return;
     } else {
-        for (int i = 0; i < len; ++i)
+        int i;
+        for (i = 0; i < len && uchar(a[i]) < 0x80U; ++i)
             *out++ = QLatin1Char(a[i]);
+        if (i == len)
+            return;
+        a += i;
+        len -= i;
     }
+
+    // we need to complement with UTF-8 appending
+    QString tmp = QString::fromUtf8(a, len);
+    memcpy(out, reinterpret_cast<const char *>(tmp.constData()), sizeof(QChar) * tmp.size());
+    out += tmp.size();
 }
 
 QT_END_NAMESPACE
diff --git a/src/corelib/tools/qstringbuilder.h b/src/corelib/tools/qstringbuilder.h
index 9b1cd1ee7e..6b258eb09a 100644
--- a/src/corelib/tools/qstringbuilder.h
+++ b/src/corelib/tools/qstringbuilder.h
@@ -381,15 +381,11 @@ operator+(const A &a, const B &b)
 }
 #endif
 
+namespace QtStringBuilder {
 template <typename A, typename B>
-QByteArray &operator+=(QByteArray &a, const QStringBuilder<A, B> &b)
+QByteArray &appendToByteArray(QByteArray &a, const QStringBuilder<A, B> &b, char)
 {
-#ifndef QT_NO_CAST_TO_ASCII
-    if (sizeof(typename QConcatenable< QStringBuilder<A, B> >::ConvertTo::value_type) == sizeof(QChar)) {
-        //it is not save to optimize as in utf8 it is not possible to compute the size
-        return a += QString(b);
-    }
-#endif
+    // append 8-bit data to a byte array
     int len = a.size() + QConcatenable< QStringBuilder<A, B> >::size(b);
     a.reserve(len);
     char *it = a.data() + a.size();
@@ -398,6 +394,23 @@ QByteArray &operator+=(QByteArray &a, const QStringBuilder<A, B> &b)
     return a;
 }
 
+#ifndef QT_NO_CAST_TO_ASCII
+template <typename A, typename B>
+QByteArray &appendToByteArray(QByteArray &a, const QStringBuilder<A, B> &b, QChar)
+{
+    // append UTF-16 data to the byte array
+    return a += QString(b);
+}
+#endif
+}
+
+template <typename A, typename B>
+QByteArray &operator+=(QByteArray &a, const QStringBuilder<A, B> &b)
+{
+    return QtStringBuilder::appendToByteArray(a, b,
+                                              typename QConcatenable< QStringBuilder<A, B> >::ConvertTo::value_type());
+}
+
 template <typename A, typename B>
 QString &operator+=(QString &a, const QStringBuilder<A, B> &b)
 {
diff --git a/tests/auto/corelib/tools/qstringbuilder/qstringbuilder1/stringbuilder.cpp b/tests/auto/corelib/tools/qstringbuilder/qstringbuilder1/stringbuilder.cpp
index d0f82a38b7..8955da97fa 100644
--- a/tests/auto/corelib/tools/qstringbuilder/qstringbuilder1/stringbuilder.cpp
+++ b/tests/auto/corelib/tools/qstringbuilder/qstringbuilder1/stringbuilder.cpp
@@ -45,7 +45,7 @@
 #define LITERAL_LEN (sizeof(LITERAL)-1)
 #define LITERAL_EXTRA "some literal" "EXTRA"
 
-// "some literal", but replacing all vocals by their umlauted UTF-8 string :)
+// "some literal", but replacing all vowels by their umlauted UTF-8 string :)
 #define UTF8_LITERAL "s\xc3\xb6m\xc3\xab l\xc3\xaft\xc3\xabr\xc3\xa4l"
 #define UTF8_LITERAL_LEN (sizeof(UTF8_LITERAL)-1)
 #define UTF8_LITERAL_EXTRA "s\xc3\xb6m\xc3\xab l\xc3\xaft\xc3\xabr\xc3\xa4l" "EXTRA"
@@ -131,14 +131,6 @@ void runScenario()
     r = string P ba;
     QCOMPARE(r, r2);
 
-#if 0
-    // now test with codec for C strings set
-    // TODO: to be re-enabled once strings default to utf8, in place of the
-    // latin1 code above.
-    QTextCodec::setCodecForCStrings(QTextCodec::codecForName("UTF-8"));
-    QVERIFY(QTextCodec::codecForCStrings());
-    QCOMPARE(QTextCodec::codecForCStrings()->name(), QByteArray("UTF-8"));
-
     string = QString::fromUtf8(UTF8_LITERAL);
     ba = UTF8_LITERAL;
 
@@ -157,7 +149,6 @@ void runScenario()
     QCOMPARE(r, r3);
     r = string P ba;
     QCOMPARE(r, r3);
-#endif
 
     ba = QByteArray(); // empty
     r = ba P string;
@@ -217,11 +208,8 @@ void runScenario()
         str +=  QLatin1String(LITERAL) P str;
         QCOMPARE(str, QString::fromUtf8(UTF8_LITERAL LITERAL UTF8_LITERAL));
 #ifndef QT_NO_CAST_FROM_ASCII
-#if 0
-        // TODO: this relies on strings defaulting to utf8, so disable this for now.
         str = (QString::fromUtf8(UTF8_LITERAL) += QLatin1String(LITERAL) P UTF8_LITERAL);
         QCOMPARE(str, QString::fromUtf8(UTF8_LITERAL LITERAL UTF8_LITERAL));
-#endif
 #endif
     }
 
@@ -237,13 +225,10 @@ void runScenario()
         ba2 += ba2 P withZero;
         QCOMPARE(ba2, QByteArray(withZero + withZero + withZero));
 #ifndef QT_NO_CAST_TO_ASCII
-#if 0
-        // TODO: this relies on strings defaulting to utf8, so disable this for now.
         ba = UTF8_LITERAL;
         ba2 = (ba += QLatin1String(LITERAL) + QString::fromUtf8(UTF8_LITERAL));
         QCOMPARE(ba2, ba);
         QCOMPARE(ba, QByteArray(UTF8_LITERAL LITERAL UTF8_LITERAL));
-#endif
 #endif
     }