QObjectPrivate: use ConnectionList's constructor instead of copy-assignment

For all new elements, this is the correct thing to do. This seems to
work around an MSVC compiler bug on ARM64. It also seems to generate
better code for x86-64 too, as a nice bonus.

See: https://developercommunity.visualstudio.com/t/codegen-elides-initializers-when-copying/10004323
Before: https://msvc.godbolt.org/z/Wcd4haaPd
After: https://msvc.godbolt.org/z/vWYjazWGr, https://gcc.godbolt.org/z/hdsvTq9nE

Fixes: QTBUG-102246
Pick-to: 6.2 6.3
Change-Id: I29f1c141c0f7436393d9fffd16e2bbbf4c0fe54d
Reviewed-by: Marc Mutz <marc.mutz@qt.io>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/corelib/kernel/qobject_p.h b/src/corelib/kernel/qobject_p.h
index d98c7065f0..c77bdeff0d 100644
--- a/src/corelib/kernel/qobject_p.h
+++ b/src/corelib/kernel/qobject_p.h
@@ -319,11 +319,14 @@ public:
             SignalVector *newVector = reinterpret_cast<SignalVector *>(malloc(sizeof(SignalVector) + (size + 1) * sizeof(ConnectionList)));
             int start = -1;
             if (vector) {
+                // not (yet) existing trait:
+                //static_assert(std::is_relocatable_v<SignalVector>);
+                //static_assert(std::is_relocatable_v<ConnectionList>);
                 memcpy(newVector, vector, sizeof(SignalVector) + (vector->allocated + 1) * sizeof(ConnectionList));
                 start = vector->count();
             }
             for (int i = start; i < int(size); ++i)
-                newVector->at(i) = ConnectionList();
+                new (&newVector->at(i)) ConnectionList();
             newVector->next = nullptr;
             newVector->allocated = size;