Ensure left/right/mid behave in a compatible way

QString and QStringRef did bounds checking for left/right/mid, whereas
QStringView was asserting on out of bounds.

Relax the behavior for QStringView and do bounds checking on pos/n
as well. This removes a source of potentially hidden errors when porting
from QStringRef (or QString) to QStringView.

Unfortunately, one difference remains, where QByteArray::left/right()
behaves differently (and somewhat more sane) than QString and
QStringRef. We're keeping the difference here, as it has been around
for many years.

Mark left/right/mid as obsolete and to be replaced with the new
first/last/slice methods.

Change-Id: I18c203799ba78c928a4610a6038089f27696c22e
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

9 files changed, 235 insertions, 125 deletions

diff --git a/src/corelib/text/qbytearray.cpp b/src/corelib/text/qbytearray.cpp
index 008ce7298b..3f83e36d90 100644
--- a/src/corelib/text/qbytearray.cpp
+++ b/src/corelib/text/qbytearray.cpp
@@ -2914,13 +2914,17 @@ bool QByteArray::isLower() const
     Returns a byte array that contains the first \a len bytes of this byte
     array.
 
+    \obsolete Use first() instead in new code.
+
     The entire byte array is returned if \a len is greater than
     size().
 
+    Returns an empty QByteArray if \a len is smaller than 0.
+
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 27
 
-    \sa startsWith(), right(), mid(), chopped(), chop(), truncate()
+    \sa first(), last(), startsWith(), chopped(), chop(), truncate()
 */
 
 QByteArray QByteArray::left(int len)  const
@@ -2935,15 +2939,18 @@ QByteArray QByteArray::left(int len)  const
 /*!
     Returns a byte array that contains the last \a len bytes of this byte array.
 
+    \obsolete Use last() instead in new code.
+
     The entire byte array is returned if \a len is greater than
     size().
 
+    Returns an empty QByteArray if \a len is smaller than 0.
+
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 28
 
-    \sa endsWith(), left(), mid(), chopped(), chop(), truncate()
+    \sa endsWith(), last(), first(), slice(), chopped(), chop(), truncate()
 */
-
 QByteArray QByteArray::right(int len) const
 {
     if (len >= size())
@@ -2957,6 +2964,8 @@ QByteArray QByteArray::right(int len) const
     Returns a byte array containing \a len bytes from this byte array,
     starting at position \a pos.
 
+    \obsolete Use slice() instead in new code.
+
     If \a len is -1 (the default), or \a pos + \a len >= size(),
     returns a byte array containing all bytes starting at position \a
     pos until the end of the byte array.
@@ -2964,13 +2973,15 @@ QByteArray QByteArray::right(int len) const
     Example:
     \snippet code/src_corelib_text_qbytearray.cpp 29
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 QByteArray QByteArray::mid(int pos, int len) const
 {
+    qsizetype p = pos;
+    qsizetype l = len;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(size(), &pos, &len)) {
+    switch (QContainerImplHelper::mid(size(), &p, &l)) {
     case QContainerImplHelper::Null:
         return QByteArray();
     case QContainerImplHelper::Empty:
@@ -2982,7 +2993,7 @@ QByteArray QByteArray::mid(int pos, int len) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QByteArray(d.data() + pos, len);
+        return QByteArray(d.data() + p, l);
     }
     Q_UNREACHABLE();
     return QByteArray();
diff --git a/src/corelib/text/qstring.cpp b/src/corelib/text/qstring.cpp
index 6707cc580e..5e3debcc34 100644
--- a/src/corelib/text/qstring.cpp
+++ b/src/corelib/text/qstring.cpp
@@ -4530,12 +4530,14 @@ QString QString::section(const QRegularExpression &re, int start, int end, Secti
     Returns a substring that contains the \a n leftmost characters
     of the string.
 
+    \obsolete Use first() instead in new code.
+
     The entire string is returned if \a n is greater than or equal
     to size(), or less than zero.
 
     \snippet qstring/main.cpp 31
 
-    \sa right(), mid(), startsWith(), chopped(), chop(), truncate()
+    \sa first(), last(), startsWith(), chopped(), chop(), truncate()
 */
 QString QString::left(int n)  const
 {
@@ -4548,12 +4550,14 @@ QString QString::left(int n)  const
     Returns a substring that contains the \a n rightmost characters
     of the string.
 
+    \obsolete Use last() instead in new code.
+
     The entire string is returned if \a n is greater than or equal
     to size(), or less than zero.
 
     \snippet qstring/main.cpp 48
 
-    \sa left(), mid(), endsWith(), chopped(), chop(), truncate()
+    \sa endsWith(), last(), first(), slice(), chopped(), chop(), truncate()
 */
 QString QString::right(int n) const
 {
@@ -4566,6 +4570,8 @@ QString QString::right(int n) const
     Returns a string that contains \a n characters of this string,
     starting at the specified \a position index.
 
+    \obsolete Use slice() instead in new code.
+
     Returns a null string if the \a position index exceeds the
     length of the string. If there are less than \a n characters
     available in the string starting at the given \a position, or if
@@ -4576,13 +4582,15 @@ QString QString::right(int n) const
 
     \snippet qstring/main.cpp 34
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 QString QString::mid(int position, int n) const
 {
+    qsizetype p = position;
+    qsizetype l = n;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(size(), &position, &n)) {
+    switch (QContainerImplHelper::mid(size(), &p, &l)) {
     case QContainerImplHelper::Null:
         return QString();
     case QContainerImplHelper::Empty:
@@ -4594,7 +4602,7 @@ QString QString::mid(int position, int n) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QString(constData() + position, n);
+        return QString(constData() + p, l);
     }
     Q_UNREACHABLE();
     return QString();
@@ -9375,28 +9383,17 @@ QString &QString::setRawData(const QChar *unicode, int size)
     \sa crbegin(), rend(), cend()
 */
 
-/*! \fn QLatin1String QLatin1String::mid(int start) const
-    \since 5.8
-
-    Returns the substring starting at position \a start in this object,
-    and extending to the end of the string.
-
-    \note This function performs no error checking.
-    The behavior is undefined when \a start < 0 or \a start > size().
-
-    \sa left(), right(), chopped(), chop(), truncate()
-*/
-
 /*! \fn QLatin1String QLatin1String::mid(int start, int length) const
     \since 5.8
-    \overload
 
     Returns the substring of length \a length starting at position
     \a start in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a start < 0, \a length < 0,
-    or \a start + \a length > size().
+    Returns a null string if the \a start index exceeds the
+    length of the string. If there are less than \a length characters
+    available in the string starting at \a start, or if
+    \a length is negative (default), the function returns all characters
+    that are available from \a start.
 
     \sa left(), right(), chopped(), chop(), truncate()
 */
@@ -9407,8 +9404,8 @@ QString &QString::setRawData(const QChar *unicode, int size)
     Returns the substring of length \a length starting at position
     0 in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
     \sa mid(), right(), chopped(), chop(), truncate()
 */
@@ -9419,8 +9416,8 @@ QString &QString::setRawData(const QChar *unicode, int size)
     Returns the substring of length \a length starting at position
     size() - \a length in this object.
 
-    \note This function performs no error checking.
-    The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
     \sa mid(), left(), chopped(), chop(), truncate()
 */
@@ -10777,8 +10774,10 @@ QStringRef QString::rightRef(int n) const
 */
 QStringRef QStringRef::mid(int pos, int n) const
 {
+    qsizetype p = pos;
+    qsizetype l = n;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(m_size, &pos, &n)) {
+    switch (QContainerImplHelper::mid(m_size, &p, &l)) {
     case QContainerImplHelper::Null:
         return QStringRef();
     case QContainerImplHelper::Empty:
@@ -10786,7 +10785,7 @@ QStringRef QStringRef::mid(int pos, int n) const
     case QContainerImplHelper::Full:
         return *this;
     case QContainerImplHelper::Subset:
-        return QStringRef(m_string, pos + m_position, n);
+        return QStringRef(m_string, p + m_position, l);
     }
     Q_UNREACHABLE();
     return QStringRef();
diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index c0b23c9733..efd57328bb 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -179,14 +179,26 @@ public:
     const_reverse_iterator rend() const noexcept { return const_reverse_iterator(begin()); }
     const_reverse_iterator crend() const noexcept { return const_reverse_iterator(begin()); }
 
-    Q_DECL_CONSTEXPR QLatin1String mid(int pos) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(pos <= size()), QLatin1String(m_data + pos, m_size - pos); }
-    Q_DECL_CONSTEXPR QLatin1String mid(int pos, int n) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(n >= 0), Q_ASSERT(pos + n <= size()), QLatin1String(m_data + pos, n); }
-    Q_DECL_CONSTEXPR QLatin1String left(int n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data, n); }
-    Q_DECL_CONSTEXPR QLatin1String right(int n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data + m_size - n, n); }
+    constexpr QLatin1String mid(int pos, int n = -1) const
+    {
+        qsizetype p = pos;
+        qsizetype l = n;
+        using namespace QtPrivate;
+        auto result = QContainerImplHelper::mid(size(), &p, &l);
+        return result == QContainerImplHelper::Null ? QLatin1String() : QLatin1String(m_data + p, l);
+    }
+    constexpr QLatin1String left(int n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QLatin1String(m_data, n);
+    }
+    constexpr QLatin1String right(int n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QLatin1String(m_data + m_size - n, n);
+    }
     Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QLatin1String chopped(int n) const
     { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QLatin1String(m_data, m_size - n); }
 
diff --git a/src/corelib/text/qstringview.cpp b/src/corelib/text/qstringview.cpp
index b9dc9d2d41..42d18851c7 100644
--- a/src/corelib/text/qstringview.cpp
+++ b/src/corelib/text/qstringview.cpp
@@ -611,49 +611,48 @@ QT_BEGIN_NAMESPACE
 */
 
 /*!
-    \fn QStringView QStringView::mid(qsizetype start) const
-
-    Returns the substring starting at position \a start in this object,
-    and extending to the end of the string.
-
-    \note The behavior is undefined when \a start < 0 or \a start > size().
-
-    \sa left(), right(), chopped(), chop(), truncate()
-*/
-
-/*!
     \fn QStringView QStringView::mid(qsizetype start, qsizetype length) const
-    \overload
 
     Returns the substring of length \a length starting at position
     \a start in this object.
 
-    \note The behavior is undefined when \a start < 0, \a length < 0,
-    or \a start + \a length > size().
+    \obsolete Use slice() instead in new code.
 
-    \sa left(), right(), chopped(), chop(), truncate()
+    Returns an empty string view if \a start exceeds the
+    length of the string. If there are less than \a length characters
+    available in the string starting at \a start, or if
+    \a length is negative (default), the function returns all characters that
+    are available from \a start.
+
+    \sa first(), last(), slice(), chopped(), chop(), truncate()
 */
 
 /*!
     \fn QStringView QStringView::left(qsizetype length) const
 
+    \obsolete Use first() instead in new code.
+
     Returns the substring of length \a length starting at position
     0 in this object.
 
-    \note The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
-    \sa mid(), right(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), startsWith(), chopped(), chop(), truncate()
 */
 
 /*!
     \fn QStringView QStringView::right(qsizetype length) const
 
+    \obsolete Use last() instead in new code.
+
     Returns the substring of length \a length starting at position
     size() - \a length in this object.
 
-    \note The behavior is undefined when \a length < 0 or \a length > size().
+    The entire string is returned if \a length is greater than or equal
+    to size(), or less than zero.
 
-    \sa mid(), left(), chopped(), chop(), truncate()
+    \sa first(), last(), slice(), endsWith(), chopped(), chop(), truncate()
 */
 
 /*!
diff --git a/src/corelib/text/qstringview.h b/src/corelib/text/qstringview.h
index 2b085c769d..6dbec21165 100644
--- a/src/corelib/text/qstringview.h
+++ b/src/corelib/text/qstringview.h
@@ -257,14 +257,24 @@ public:
 
     Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QChar at(qsizetype n) const { return (*this)[n]; }
 
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView mid(qsizetype pos) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(pos <= size()), QStringView(m_data + pos, m_size - pos); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView mid(qsizetype pos, qsizetype n) const
-    { return Q_ASSERT(pos >= 0), Q_ASSERT(n >= 0), Q_ASSERT(pos + n <= size()), QStringView(m_data + pos, n); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView left(qsizetype n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QStringView(m_data, n); }
-    Q_REQUIRED_RESULT Q_DECL_CONSTEXPR QStringView right(qsizetype n) const
-    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QStringView(m_data + m_size - n, n); }
+    Q_REQUIRED_RESULT constexpr QStringView mid(qsizetype pos, qsizetype n = -1) const
+    {
+        using namespace QtPrivate;
+        auto result = QContainerImplHelper::mid(size(), &pos, &n);
+        return result == QContainerImplHelper::Null ? QStringView() : QStringView(m_data + pos, n);
+    }
+    Q_REQUIRED_RESULT constexpr QStringView left(qsizetype n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QStringView(m_data, n);
+    }
+    Q_REQUIRED_RESULT constexpr QStringView right(qsizetype n) const
+    {
+        if (size_t(n) >= size_t(size()))
+            n = size();
+        return QStringView(m_data + m_size - n, n);
+    }
 
     Q_REQUIRED_RESULT constexpr QStringView first(qsizetype n) const
     { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); return QStringView(m_data, int(n)); }
diff --git a/src/corelib/tools/qarraydata.cpp b/src/corelib/tools/qarraydata.cpp
index 497eae1f7f..b72ca20131 100644
--- a/src/corelib/tools/qarraydata.cpp
+++ b/src/corelib/tools/qarraydata.cpp
@@ -281,33 +281,4 @@ void QArrayData::deallocate(QArrayData *data, size_t objectSize,
     ::free(data);
 }
 
-namespace QtPrivate {
-/*!
-  \internal
-*/
-QContainerImplHelper::CutResult QContainerImplHelper::mid(int originalLength, int *_position, int *_length)
-{
-    int &position = *_position;
-    int &length = *_length;
-    if (position > originalLength)
-        return Null;
-
-    if (position < 0) {
-        if (length < 0 || length + position >= originalLength)
-            return Full;
-        if (length + position <= 0)
-            return Null;
-        length += position;
-        position = 0;
-    } else if (uint(length) > uint(originalLength - position)) {
-        length = originalLength - position;
-    }
-
-    if (position == 0 && length == originalLength)
-        return Full;
-
-    return length > 0 ? Subset : Empty;
-}
-}
-
 QT_END_NAMESPACE
diff --git a/src/corelib/tools/qarraydata.h b/src/corelib/tools/qarraydata.h
index b6fc05e108..de52713348 100644
--- a/src/corelib/tools/qarraydata.h
+++ b/src/corelib/tools/qarraydata.h
@@ -306,7 +306,37 @@ namespace QtPrivate {
 struct Q_CORE_EXPORT QContainerImplHelper
 {
     enum CutResult { Null, Empty, Full, Subset };
-    static CutResult mid(int originalLength, int *position, int *length);
+    static constexpr CutResult mid(qsizetype originalLength, qsizetype *_position, qsizetype *_length)
+    {
+        qsizetype &position = *_position;
+        qsizetype &length = *_length;
+        if (position > originalLength) {
+            position = 0;
+            length = 0;
+            return Null;
+        }
+
+        if (position < 0) {
+            if (length < 0 || length + position >= originalLength) {
+                position = 0;
+                length = originalLength;
+                return Full;
+            }
+            if (length + position <= 0) {
+                position = length = 0;
+                return Null;
+            }
+            length += position;
+            position = 0;
+        } else if (size_t(length) > size_t(originalLength - position)) {
+            length = originalLength - position;
+        }
+
+        if (position == 0 && length == originalLength)
+            return Full;
+
+        return length > 0 ? Subset : Empty;
+    }
 };
 }
 
diff --git a/src/corelib/tools/qvector.h b/src/corelib/tools/qvector.h
index c2cdcba41d..b502a86442 100644
--- a/src/corelib/tools/qvector.h
+++ b/src/corelib/tools/qvector.h
@@ -710,8 +710,10 @@ int QVector<T>::lastIndexOf(const T &t, int from) const noexcept
 template <typename T>
 inline QVector<T> QVector<T>::mid(int pos, int len) const
 {
+    qsizetype p = pos;
+    qsizetype l = len;
     using namespace QtPrivate;
-    switch (QContainerImplHelper::mid(d.size, &pos, &len)) {
+    switch (QContainerImplHelper::mid(d.size, &p, &l)) {
     case QContainerImplHelper::Null:
     case QContainerImplHelper::Empty:
         return QVector();
@@ -722,8 +724,8 @@ inline QVector<T> QVector<T>::mid(int pos, int len) const
     }
 
     // Allocate memory
-    DataPointer copied(Data::allocate(len));
-    copied->copyAppend(constBegin() + pos, constBegin() + pos + len);
+    DataPointer copied(Data::allocate(l));
+    copied->copyAppend(constBegin() + p, constBegin() + p + l);
     return copied;
 }
 
diff --git a/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp b/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
index ff616f2334..ab0d5f3749 100644
--- a/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
+++ b/tests/auto/corelib/text/qstringapisymmetry/tst_qstringapisymmetry.cpp
@@ -644,9 +644,6 @@ private:
     void chop_data();
     template <typename String> void chop_impl();
 
-    void truncate_data() { left_data(); }
-    template <typename String> void truncate_impl();
-
 private Q_SLOTS:
 
     void mid_QString_data() { mid_data(); }
@@ -660,16 +657,16 @@ private Q_SLOTS:
     void mid_QByteArray_data() { mid_data(); }
     void mid_QByteArray() { mid_impl<QByteArray>(); }
 
-    void left_truncate_QString_data() { left_data(); }
-    void left_truncate_QString() { left_impl<QString>(); }
-    void left_truncate_QStringRef_data() { left_data(); }
-    void left_truncate_QStringRef() { left_impl<QStringRef>(); }
-    void left_truncate_QStringView_data() { left_data(); }
-    void left_truncate_QStringView() { left_impl<QStringView>(); }
-    void left_truncate_QLatin1String_data() { left_data(); }
-    void left_truncate_QLatin1String() { left_impl<QLatin1String>(); }
-    void left_truncate_QByteArray_data() { left_data(); }
-    void left_truncate_QByteArray() { left_impl<QByteArray>(); }
+    void left_QString_data() { left_data(); }
+    void left_QString() { left_impl<QString>(); }
+    void left_QStringRef_data() { left_data(); }
+    void left_QStringRef() { left_impl<QStringRef>(); }
+    void left_QStringView_data() { left_data(); }
+    void left_QStringView() { left_impl<QStringView>(); }
+    void left_QLatin1String_data() { left_data(); }
+    void left_QLatin1String() { left_impl<QLatin1String>(); }
+    void left_QByteArray_data();
+    void left_QByteArray() { left_impl<QByteArray>(); }
 
     void right_QString_data() { right_data(); }
     void right_QString() { right_impl<QString>(); }
@@ -679,7 +676,7 @@ private Q_SLOTS:
     void right_QStringView() { right_impl<QStringView>(); }
     void right_QLatin1String_data() { right_data(); }
     void right_QLatin1String() { right_impl<QLatin1String>(); }
-    void right_QByteArray_data() { right_data(); }
+    void right_QByteArray_data();
     void right_QByteArray() { right_impl<QByteArray>(); }
 
     void slice_QString_data() { slice_data(); }
@@ -1533,6 +1530,37 @@ void tst_QStringApiSymmetry::tok_impl() const
 void tst_QStringApiSymmetry::mid_data()
 {
     slice_data();
+
+    // mid() has a wider contract compared to slize(), so test those cases here:
+#define ROW(base, p, n, r1, r2) \
+    QTest::addRow("%s %d %d", #base, p, n) << QStringRef(&base) << QLatin1String(#base) << p << n << QStringRef(&r1) << QStringRef(&r2)
+
+    ROW(a, -1, 0, a, null);
+    ROW(a, -1, 2, a, a);
+    ROW(a, -1, 3, a, a);
+    ROW(a, 0, -1, a, a);
+    ROW(a, 0, 2, a, a);
+    ROW(a, -1, -1, a, a);
+    ROW(a, 1, -1, empty, empty);
+    ROW(a, 1, 1, empty, empty);
+    ROW(a, 2, -1, null, null);
+    ROW(a, 2, 1, null, null);
+
+    ROW(abc, -1, -1, abc, abc);
+    ROW(abc, -1, 0, abc, null);
+    ROW(abc, -1, 2, abc, a);
+    ROW(abc, -1, 3, abc, ab);
+    ROW(abc, -1, 5, abc, abc);
+    ROW(abc, 0, -1, abc, abc);
+    ROW(abc, 0, 5, abc, abc);
+    ROW(abc, -1, 1, abc, null);
+    ROW(abc, -1, 2, abc, a);
+    ROW(abc, -1, 4, abc, abc);
+    ROW(abc, 1, -1, bc, bc);
+    ROW(abc, 1, 1, bc, b);
+    ROW(abc, 3, -1, empty, empty);
+    ROW(abc, 3, 1, empty, empty);
+#undef ROW
 }
 
 template <typename String>
@@ -1578,6 +1606,35 @@ void tst_QStringApiSymmetry::mid_impl()
 void tst_QStringApiSymmetry::left_data()
 {
     first_data();
+
+    // specific data testing out of bounds cases
+#define ROW(base, n, res) \
+    QTest::addRow("%s%d", #base, n) << QStringRef(&base) << QLatin1String(#base) << n << QStringRef(&res);
+
+    ROW(a, -1, a);
+    ROW(a, 2, a);
+
+    ROW(ab, -100, ab);
+    ROW(ab, 100, ab);
+#undef ROW
+}
+
+// This is different from the rest for historical reasons. As we're replacing
+// left() with first() as the recommended API, there's no point fixing this anymore
+void tst_QStringApiSymmetry::left_QByteArray_data()
+{
+    first_data();
+
+    // specific data testing out of bounds cases
+#define ROW(base, n, res) \
+    QTest::addRow("%s%d", #base, n) << QStringRef(&base) << QLatin1String(#base) << n << QStringRef(&res);
+
+    ROW(a, -1, empty);
+    ROW(a, 2, a);
+
+    ROW(ab, -100, empty);
+    ROW(ab, 100, ab);
+#undef ROW
 }
 
 template <typename String>
@@ -1606,19 +1663,40 @@ void tst_QStringApiSymmetry::left_impl()
         QCOMPARE(left.isNull(), result.isNull());
         QCOMPARE(left.isEmpty(), result.isEmpty());
     }
-    {
-        auto left = s;
-        left.truncate(n);
-
-        QCOMPARE(left, result);
-        QCOMPARE(left.isNull(), result.isNull());
-        QCOMPARE(left.isEmpty(), result.isEmpty());
-    }
 }
 
 void tst_QStringApiSymmetry::right_data()
 {
     last_data();
+
+    // specific data testing out of bounds cases
+#define ROW(base, n, res) \
+    QTest::addRow("%s%d", #base, n) << QStringRef(&base) << QLatin1String(#base) << n << QStringRef(&res);
+
+    ROW(a, -1, a);
+    ROW(a, 2, a);
+
+    ROW(ab, -100, ab);
+    ROW(ab, 100, ab);
+#undef ROW
+}
+
+// This is different from the rest for historical reasons. As we're replacing
+// left() with first() as the recommended API, there's no point fixing this anymore
+void tst_QStringApiSymmetry::right_QByteArray_data()
+{
+    last_data();
+
+    // specific data testing out of bounds cases
+#define ROW(base, n, res) \
+    QTest::addRow("%s%d", #base, n) << QStringRef(&base) << QLatin1String(#base) << n << QStringRef(&res);
+
+    ROW(a, -1, empty);
+    ROW(a, 2, a);
+
+    ROW(ab, -100, empty);
+    ROW(ab, 100, ab);
+#undef ROW
 }
 
 template <typename String>
@@ -1661,8 +1739,6 @@ void tst_QStringApiSymmetry::slice_data()
 //    QTest::addRow("null") << QStringRef() << QLatin1String() << 0 << 0 << QStringRef() << QStringRef();
     QTest::addRow("empty") << QStringRef(&empty) << QLatin1String("") << 0 << 0 << QStringRef(&empty) << QStringRef(&empty);
 
-    // Some classes' mid() implementations have a wide contract, others a narrow one
-    // so only test valid arguents here:
 #define ROW(base, p, n, r1, r2) \
     QTest::addRow("%s%d%d", #base, p, n) << QStringRef(&base) << QLatin1String(#base) << p << n << QStringRef(&r1) << QStringRef(&r2)