Compiler returns "undefined symbol error" on access of symbol that's only defined in an interface

SDK: 8.1.1

typedef IFoo as interface {
    var bar as Number; // NOTE: bar isn't defined as a symbol anywhere else in the program
};

function baz(foo as IFoo) as Void {
    System.println("bar = " + foo.bar); // error: "Undefined symbol ':bar' detected"
}

I suppose this bug is ultimately harmless as there's no use in defining and using an interface if no class implements it. And if a class implements an interface, then all the symbols in that interface will be defined in the class.