Must the offset be unknown in order for the alias analysis to work? I think of dynamic_cast<D*>(b) as meaning roughly (*b instanceof D) ? static_cast<D*>(b) : nullptr, but maybe that's an oversimplification? If the type check doesn't pass, then the returned pointer doesn't point to any object, but that's also sort of true for a static_cast.

The "sidecast" case in https://en.cppreference.com/w/cpp/language/dynamic_cast means that the offset is at least sometimes unknown. Even for the downcast case, there isn't an unambiguous offset for a conversion from Y to Base in https://godbolt.org/z/ebWA7x

There's also the special case of dynamic_cast<void*> which casts to the most specific polymorphic class: https://godbolt.org/z/AZJpxS

Thanks for the explanations. Inheritance is scary!