private static interface FuncToTFunc<T> {
          Func<T> apply(FuncToTFunc<T> x);
      }

Also is Func defined mutually recursively with FuncToFunc in excerpt below?

      public static <T> Func<T> Y(final Func<Func<T>> r) {
        return ((FuncToTFunc<T>) f -> f.apply(f))
            .apply(
                f -> r.apply(
                    x -> f.apply(f).apply(x)));
      }

FuncToFunc are defined recursively.

Interface Func doesn't refer to FuncToFunc in its declaration, and interface FuncToFunc doesn't refer to Func. The method Y does refer to those two interfaces, but it's declared later than those are, and they don't (and can't, acvtually) refer to it.