I'm beginning to see that now. In my test case the situation has been much confused by the fact that b1 has/didn't have a virtual destructor which in turn makes it possible/impossible. Oh and running on different compilers in debug/release mode changed behavior as well which left me totally confused. So I'm working through my confusion.

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I wrote that very thing a year or so ago for the purpose of testing if a base pointer type can be statically downcast to a derived type.

My problem exactly !

I should warn you that this isn't quite enough information to test for static downcastability in all cases. Take the following example:

struct b1 {}; struct b2 : b1 {}; struct b3 : virtual b2 {};

Here, b1 is not a virtual base of b3, but a downcast from b1* to b3* is still impossible. I don't believe there is any way to test for this case without compiler assistance.

Robert Ramey