Do not shift the data pointer when removing all elements from QList

Because leaving the pointer untouched is a much more expected behavior

The tests for this (and not only) logic can be found in the following commit

Change-Id: Iec9eec9bbce04c9fd90cb6be9627c135cd989b7f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit 38bba2a87c6c5c2b8100870add6d0d7ad559e669)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/corelib/tools/qarraydataops.h b/src/corelib/tools/qarraydataops.h
index b70e8e4d9a..cffe1b4575 100644
--- a/src/corelib/tools/qarraydataops.h
+++ b/src/corelib/tools/qarraydataops.h
@@ -231,10 +231,12 @@ public:
         // are invalidated. However, erasing from the beginning effectively
         // means that all iterators are invalidated. We can use this freedom to
         // erase by moving towards the end.
-        if (b == this->begin())
+        if (b == this->begin() && e != this->end()) {
             this->ptr = e;
-        else if (e != this->end())
-            ::memmove(static_cast<void *>(b), static_cast<void *>(e), (static_cast<T *>(this->end()) - e) * sizeof(T));
+        } else if (e != this->end()) {
+            ::memmove(static_cast<void *>(b), static_cast<void *>(e),
+                      (static_cast<T *>(this->end()) - e) * sizeof(T));
+        }
         this->size -= n;
     }
 
@@ -610,7 +612,7 @@ public:
         // are invalidated. However, erasing from the beginning effectively
         // means that all iterators are invalidated. We can use this freedom to
         // erase by moving towards the end.
-        if (b == this->begin()) {
+        if (b == this->begin() && e != this->end()) {
             this->ptr = e;
         } else {
             const T *const end = this->end();
@@ -840,7 +842,7 @@ public:
         // erase by moving towards the end.
 
         std::destroy(b, e);
-        if (b == this->begin()) {
+        if (b == this->begin() && e != this->end()) {
             this->ptr = e;
         } else if (e != this->end()) {
             memmove(static_cast<void *>(b), static_cast<const void *>(e), (static_cast<const T *>(this->end()) - e)*sizeof(T));