Some inner classes can occur in static contexts (8.4.1)—e.g., a local class declared in a static method. But these inner classes have no immediately enclosing instance, so shouldn't have a receiver parameter.

Proposed rule:

A receiver parameter may appear only in the FormalParameterList of an instance method or ~~an inner class's constructor~~ **a constructor of an inner class, where the inner class is not declared in a static context**; otherwise, a compile-time error occurs.

Or, if that's too much of a mouthful:

A receiver parameter may appear only in the FormalParameterList of an instance method or an inner class's constructor; otherwise, a compile-time error occurs.

**If a receiver parameter appears in the FormalParameterList of an inner class's constructor, but that class is declared in a static context, a compile-time error occurs.**