Change QLineF::setLength() to work whenever length() is non-zero

Previously it only worked when isNull() was false, which is true for
very short lines, even though length() may be non-zero.

[ChangeLog][QtCore][QLineF] QLineF::setLength() will now set the
length if the line's length() is non-zero. Previously, it was
documented to only set the length if isNull() was false; this is a
fuzzy check, so isNull() could be true for a line with non-zero
length().

Fixes: QTBUG-89569
Change-Id: I803e622ad09c85815dde25df8dd3ba6dfcba0714
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 6974737695eae5a41bc8a3f344a4f1f199006f21)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 11 insertions, 11 deletions

diff --git a/src/corelib/tools/qline.cpp b/src/corelib/tools/qline.cpp
index ebcbba0195..1eb645501c 100644
--- a/src/corelib/tools/qline.cpp
+++ b/src/corelib/tools/qline.cpp
@@ -517,12 +517,13 @@ QDataStream &operator>>(QDataStream &stream, QLine &line)
     \fn void QLineF::setLength(qreal length)
 
     Sets the length of the line to the given \a length. QLineF will
-    move the end point - p2() - of the line to give the line its new length.
+    move the end point - p2() - of the line to give the line its new
+    length, unless length() was previously zero, in which case no
+    scaling is attempted. For lines with very short lengths
+    (represented by denormal floating-point values), results may be
+    imprecise.
 
-    A null line will not be rescaled. For non-null lines with very short lengths
-    (represented by denormal floating-point values), results may be imprecise.
-
-    \sa length(), isNull(), unitVector()
+    \sa length(), unitVector()
 */
 
 /*!
diff --git a/src/corelib/tools/qline.h b/src/corelib/tools/qline.h
index 4948559509..9b544f2e7e 100644
--- a/src/corelib/tools/qline.h
+++ b/src/corelib/tools/qline.h
@@ -372,12 +372,11 @@ constexpr inline QPointF QLineF::center() const
 
 inline void QLineF::setLength(qreal len)
 {
-    if (isNull())
-        return;
-    Q_ASSERT(length() > 0);
-    const QLineF v = unitVector();
-    len /= v.length(); // In case it's not quite exactly 1.
-    pt2 = QPointF(pt1.x() + len * v.dx(), pt1.y() + len * v.dy());
+    const qreal oldLength = length();
+    // Scale len by dx() / length() and dy() / length(), two O(1) quantities,
+    // rather than scaling dx() and dy() by len / length(), which might overflow.
+    if (oldLength > 0)
+        pt2 = QPointF(pt1.x() + len * (dx() / oldLength), pt1.y() + len * (dy() / oldLength));
 }
 
 constexpr inline QPointF QLineF::pointAt(qreal t) const