gistfile1.adb

-- compilation unit 1:

generic
   type T is private;
package Unit is
   -- ...
end Unit;

-- compilation unit 2:

generic
   type T2 is private;
package Unit.Child is
   -- ...
end Unit.Child;

-- compilation unit 3:

with Unit.Child; -- neccessary, but GNAT says it's unreferenced.

generic
   type T2 is private;
package Specialization is
   package Base is new Unit (T2);
   
   generic package Child renames Base.Child;
end Specialization;

--  compilation unit 4:

with Specialization;

procedure Test is
   package Instance is new Specialization;
   package Instance_Child is new Instance.Child;
   -- if I didn't rename the generic package, I'd need to do:
   -- package Instance_Child is new Instance.Base.Child;
begin
   null;
end Test;