The Swift class model does not support overriding declarations where either
the overridden declaration or the overriding declaration are in an extension.
However, the Objective-C class model does, so marking the declaration as
@objc (when possible) will work around the limitation.
Customize the "cannot override declaration in extension" diagnostic to
suggest adding @objc to the overridden declaration in cases where
@objc is permitted. Fixes SR-6512 / rdar://problem/35787914.
Previous versions of Swift accidentally treated lazy properties as
computed properties because of how they were implemented. Now that we
check this correctly, we've broken source compatibility. Downgrade the
error to a warning in this case.
(Arguably we could /allow/ overriding with a stored property. The
original concerns were that you could accidentally end up with extra
storage you didn't need, and that observing accessors would behave
differently based on whether or not the property was overriding. But
there's at least no ambiguity for 'lazy', which can't have observing
accessors today.)
rdar://problem/35870371
