multiple inheritance bug

[Martin v. Loewis]

>   delete b;                // segmentation fault

I assume you consider this segmentation fault to be a bug in g++.  It
is not, this is a bug in your program. 

You are invoking delete on an object whose static type differs from
its dynamic type; yet the the class has no virtual destructor. This is
undefined behaviour: it may work (as it does for A*), it may crash, or
it may toast your computer.

To correct this error, change A and B to
struct A {
 virtual ~A(){}
};

struct B {
 virtual ~B(){}
};

Hope this helps,
Martin