Isn't that case already covered by https://eel.is/c++draft/expr.dynamic.cast#3?

Only when "v is the same as T (ignoring cv-qualifications)", which might not be the case if v is a base class subobject of C.

I think it is covered by [expr.dynamic.cast#8.1], namely T is the type of the most derived object while the v is a subobject thereof.

If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.

C may be the type of the most derived object or the one base class subobject of which the v is a subobject.

Thank you, that's right.