From 6512a7fc642c65455db770385c67cfa6d71c294c Mon Sep 17 00:00:00 2001
From: Andrei Golubev <andrei.golubev@qt.io>
Date: Tue, 9 Feb 2021 14:03:04 +0100
Subject: Restore pre-Qt6 QList::fill() behavior

Somehow QList::fill(t, newSize) introduced a regression in Qt6:
when newSize < QList::size() we should resize to the newSize.
This is aligned with QVector::fill() in 5.15 and std::vector::assign()

While 6.0 is already out, picking it to 6.0.x could save someone who
haven't migrated yet as well as fix some accidental bugs in Qt's code

[ChangeLog][QtCore][QList] Fixed QList::fill() regression introduced in
6.0: calling fill() with size < current list size wouldn't truncate the
list

Fixes: QTBUG-91042
Pick-to: 6.0 6.1
Change-Id: Ic166e2c5e42390b61df1030f7c705e344433f7f2
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
---
 src/corelib/tools/qlist.h                    |  5 +-
 tests/auto/corelib/tools/qlist/tst_qlist.cpp | 70 +++++++++++++++++++++++++---
 2 files changed, 67 insertions(+), 8 deletions(-)

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 662fede6f7..b50cd9090f 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -834,8 +834,11 @@ inline QList<T> &QList<T>::fill(parameter_type t, qsizetype newSize)
         // we're detached
         const T copy(t);
         d->assign(d.begin(), d.begin() + qMin(size(), newSize), t);
-        if (newSize > size())
+        if (newSize > size()) {
             d->copyAppend(newSize - size(), copy);
+        } else if (newSize < size()) {
+            d->truncate(newSize);
+        }
     }
     return *this;
 }
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index 64123c61a7..80adb0f6a1 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -256,7 +256,9 @@ private slots:
     void fillInt() const;
     void fillMovable() const;
     void fillCustom() const;
-    void fillDetaches() const;
+    void fillDetachInt() const;
+    void fillDetachMovable() const;
+    void fillDetachCustom() const;
     void first() const;
     void fromListInt() const;
     void fromListMovable() const;
@@ -363,6 +365,7 @@ private:
     template<typename T> void erase(bool shared) const;
     template<typename T> void eraseReserved() const;
     template<typename T> void fill() const;
+    template<typename T> void fillDetach() const;
     template<typename T> void fromList() const;
     template<typename T> void insert() const;
     template<typename T> void qhash() const;
@@ -1443,6 +1446,10 @@ void tst_QList::fill() const
                                  << SimpleValue<T>::at(2) << SimpleValue<T>::at(2)
                                  << SimpleValue<T>::at(2) << SimpleValue<T>::at(2)
                                  << SimpleValue<T>::at(2) << SimpleValue<T>::at(2));
+
+    // make sure it can resize to smaller size as well
+    myvec.fill(SimpleValue<T>::at(3), 2);
+    QCOMPARE(myvec, QList<T>() << SimpleValue<T>::at(3) << SimpleValue<T>::at(3));
 }
 
 void tst_QList::fillInt() const
@@ -1464,14 +1471,63 @@ void tst_QList::fillCustom() const
     QCOMPARE(instancesCount, Custom::counter.loadAcquire());
 }
 
-void tst_QList::fillDetaches() const
+template<typename T>
+void tst_QList::fillDetach() const
 {
-    QList<int> test = { 1, 2, 3 };
-    QList<int> copy = test;
-    copy.fill(42);
+    // detaches to the same size
+    {
+        QList<T> original = { SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) };
+        QList<T> copy = original;
+        copy.fill(SimpleValue<T>::at(2));
+
+        QCOMPARE(original,
+                 QList<T>({ SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) }));
+        QCOMPARE(copy,
+                 QList<T>({ SimpleValue<T>::at(2), SimpleValue<T>::at(2), SimpleValue<T>::at(2) }));
+    }
+
+    // detaches and grows in size
+    {
+        QList<T> original = { SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) };
+        QList<T> copy = original;
+        copy.fill(SimpleValue<T>::at(2), 5);
 
-    QCOMPARE(test, QList<int>({1, 2, 3}));
-    QCOMPARE(copy, QList<int>({42, 42, 42}));
+        QCOMPARE(original,
+                 QList<T>({ SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) }));
+        QCOMPARE(copy,
+                 QList<T>({ SimpleValue<T>::at(2), SimpleValue<T>::at(2), SimpleValue<T>::at(2),
+                            SimpleValue<T>::at(2), SimpleValue<T>::at(2) }));
+    }
+
+    // detaches and shrinks in size
+    {
+        QList<T> original = { SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) };
+        QList<T> copy = original;
+        copy.fill(SimpleValue<T>::at(2), 1);
+
+        QCOMPARE(original,
+                 QList<T>({ SimpleValue<T>::at(1), SimpleValue<T>::at(1), SimpleValue<T>::at(1) }));
+        QCOMPARE(copy, QList<T>({ SimpleValue<T>::at(2) }));
+    }
+}
+
+void tst_QList::fillDetachInt() const
+{
+    fillDetach<int>();
+}
+
+void tst_QList::fillDetachMovable() const
+{
+    const int instancesCount = Movable::counter.loadAcquire();
+    fillDetach<Movable>();
+    QCOMPARE(instancesCount, Movable::counter.loadAcquire());
+}
+
+void tst_QList::fillDetachCustom() const
+{
+    const int instancesCount = Custom::counter.loadAcquire();
+    fillDetach<Custom>();
+    QCOMPARE(instancesCount, Custom::counter.loadAcquire());
 }
 
 void tst_QList::first() const
-- 
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