Re: Preventing private procedure visibility being made public through extension

[Jere <jhb.chat@gmail.com>]

On Saturday, May 20, 2017 at 4:13:10 PM UTC-4, AdaMagica wrote:
> > with Base;
> > 
> > package Derived is
> >    
> >    type Derived_Param is new Base.Base_Param with null record;
> >    
> >    type Derived_Type is new Base.Base_Type with private;
> >    
> >    procedure Something
> >       (Obj : in out Derived_Type; 
> >        Value : Derived_Param'Class)
> >    is null;
> >    
> > private
> >    
> >    type Derived_Type is new Base.Base_Type with null record;
> >    
> >    overriding
> >    procedure Something
> >       (Obj : in out Derived_Type; 
> >        Value : Base.Base_Param'Class) 
> >    is null;
> >    
> > end Derived;
> 
> Here you are wrong. Hiding the overriding does not prevent the client from calling the operation. It nearly makes no difference whether you override an inherited operation in the public or in the private part. (There are a few syntactic diffrences though.)
> 
> > *************************************************
> > 
> > This is all well and good.  Variables of Derived_Type 
> > can only call the version of something that I specified 
> > publicly and the overriden one cannot be called.
> 
> As I said, this is wrong.
> 
> > 
> > However, variables of More_Derived_Type have public 
> > visibility to both versions of Something.  I would 
> > have hoped that the version hidden by Derived_Type 
> > would keep it hidden, but extending the type makes
> > the private procedure visible again.
> > 
> > Example main:
> > 
> > main.adb
> > *************************************************
> > with Base;
> > with Derived; use Derived;
> > 
> > procedure Main is
> >    p : Derived_Param;
> >    d : Derived_Type;
> >    m : More_Derived_Type;
> > begin
> >    d.Something(p);  -- Works as expected
> 
> I think your compiler is in error here. This call is (IMHO) also ambiguous.
> Both of these operations are visible here:

Perhaps a compiler bug, but it is even more than just not 
ambiguous, it cannot call the version of the procedure that I 
put in the private section (for variables of type Derived_Type).  

If I declare a variable:

b : Base.Base_Param;

and try to call

d.Something(b);

It will fail with an error:
Expected type "Derived_Param'Class" defined at derived.ads:5
expected type "Derived_Param'Class" defined at derived.ads:5
found type "Base_Param" defined at base.ads:3

I even tried casting b to Base_Param'Class with similar 
results (compiler error).  It is acting like the 
procedure is not publicly visible for variable d.

Now if I didn't have a procedure named Something declared, 
it acts as you described.  Is this a compiler bug?