Proposed resolution (January, 2017):

Change 7.6.1.9 [expr.static.cast] paragraph 2 as follows:

...If the object of type “cv1 B” is actually a base class subobject of an object of type D, the result refers to the enclosing object of type D. Otherwise, the behavior is undefined. [Example:...

[Adopted at the February/March, 2017 meeting.]

The current wording of 7.6.1.9 [expr.static.cast] paragraph 2 appears to permit the following example:

  struct B {
    int i;
  };

  struct D : B {
    int j;
    B b;
  };

  int main() {
    D d;

    B &br = d.b;
    D &dr = static_cast<D&>(br);  // Okay?
  }

Presumably such casts should only be supported if the operand object is a base class subobject, not a member subobject.