Generic private type declaration

["Alejandro R. Mosteo" <alejandro@mosteo.com>]

Hello,

I need some eyes on this error because I'm missing something basic. When 
compiling this code:

procedure B001_Tagged is

    generic
       type X is private;
    package Untagged is

       type Y is new X;

    end Untagged;

    package Ok is new Untagged (Integer);

    type Void is tagged null record;

    package Err is new Untagged (Void); --  Error here

begin
    null;
end B001_Tagged;

I get in both gnat 4.9.3 and gpl2016 the following error:

b001_tagged.adb:15:04: instantiation error at line 7
b001_tagged.adb:15:04: type derived from tagged type must have extension
gnatmake: "b001_tagged.adb" compilation error

I would expect that the view inside the generic package is untagged and 
so the type renaming in line 7 should be correct? Or I'm floundering 
with the generic parameter declaration?

Thanks,
Alex.