Rationale (February, 2012):

9.3.4.7 [dcl.fct.default] paragraphs 8-9 should be interpreted as making the example ill-formed.

Consider:

    struct A {
       int i;
       A() { void foo(int=i); }
    };

It's not clear whether that is well-formed or not. It uses this, which might be thought of as a kind of parameter or local variable, which would make the default argument ill-formed. On the other hand, there doesn't seem to be a good reason to ban the code, either.