Refuse to relocate non-copy/move-constructible types

When a type was explicitly made non-copyable and non-movable, refuse
attempts to mark it as relocatable.

Trivial relocatability is an optimization for move+destroy, so we
shouldn't allow working around the class author's wish to have a
non-movable type by a user making it Q_RELOCATABLE_TYPE.

Therefore, add a static_assert() to Q_DECLARE_TYPEINFO that fires
when a non-copy/move-constructible type is made Q_RELOCATABLE_TYPE.

Also add a similar static assertion to QTypeInfoMerger: All members
of a class T may be Q_RELOCATABLE_TYPE. But that doesn't mean that
T itself is:

   class T {
      QString m_s;
      QByteArray m_b;
      Q_DISABLE_COPY(T)
   public:
      ~~~~
   };

so check that T is actually copyable (or movable) when QTypeInfoMerger
would have have yielded isRelocatable.

Since Q_DECLARE_TYPEINFO and QTypeInfoMerger are not the only ways in
which a user can mark a type as relocatable (manual QTypeInfo
specialization is another option, and required, for certain template
classes), also check in q_uninitialized_relocate_n().

[ChangeLog][QtCore][QTypeInfo/QTypeInfoMerger] No longer allows
marking as Q_RELOCATABLE_TYPE a type that is neither movable nor
copyable.

Change-Id: If6b3e5c1bfb6538bd280262eefbb08ba3c810e4c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/corelib/tools/qcontainertools_impl.h b/src/corelib/tools/qcontainertools_impl.h
index 40ed0a84e9..d66705ef4d 100644
--- a/src/corelib/tools/qcontainertools_impl.h
+++ b/src/corelib/tools/qcontainertools_impl.h
@@ -72,6 +72,8 @@ template <typename T, typename N>
 void q_uninitialized_relocate_n(T* first, N n, T* out)
 {
     if constexpr (QTypeInfo<T>::isRelocatable) {
+        static_assert(std::is_copy_constructible_v<T> || std::is_move_constructible_v<T>,
+                      "Refusing to relocate this non-copy/non-move-constructible type.");
         if (n != N(0)) { // even if N == 0, out == nullptr or first == nullptr are UB for memcpy()
             std::memcpy(static_cast<void *>(out),
                         static_cast<const void *>(first),