Construction from nullptr wasn't, before, because it was using the
QPointer(T*) constructor, which cannot be constexpr. Add a constexpr
QPointer(std::nullptr_t) constructor to enable this use-case.
This requires to mark the (T*) constructor as Q_WEAK_OVERLOAD,
otherwise legacy construction from a literal 0 would be ambiguous.
No documentation changes needed, as the set of valid expressions
(apart from constinit'ing) has not changed. Mention the nullptr ctor,
though, without \since.
Add a test to confirm that contruction from derived still works.
Change-Id: If9d5281f6eca0c408a69f03fecba64a70a0c9cf0
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
1219dbcd12