Package formal parameter visibility

Package formal parameter visibility

[Jere]

I ran into something that interested me while dorking around
with some Ada code.  The error makes sense when I think about it,
but I ran into what looks like an inconsistency on the surface.

Consider the package:

##################
generic
   type My_Type is limited private;
   with procedure Do_Something(Instance : in out My_Type);
package Package_A is
   subtype My_Subtype is My_Type;
end Package_A;
#####################

It doesn't do much on its own and if I try to use it like so:

#####################
with Package_A;

procedure Main is
   
   procedure Do_Something(Instance : in out Integer) is
   begin
      Instance := 23;
   end Do_Something;
   
   package A is new Package_A(My_Type      => Integer,
                              Do_Something => Do_Something);
   Var : A.My_Subtype;
begin
   A.Do_Something(Var);
end Main;
######################

I get the error: "Do_Something" is not a visible entity of "A"

Fair enough, I can't use formal parameters in a client directly.
If I want to, I have to either use some rename clauses or wrapper
procedures.

Tben I noticed I could do it if another generic package used it in this
manner:

###################################
with Package_A;

generic
   with package A is new Package_A(<>);
package Package_B is
   procedure Do_Something;
end Package_B;
###################################

###################################
package body Package_B is
   procedure Do_Something is
      Var : A.My_Subtype;
   begin
      A.Do_Something(Var);  -- Shouldn't I get the error here?
   end Do_Something;
end Package_B;
###################################

If I use A as a formal parameter into another package, I suddenly get
visibility of A's formal parameters in that new package.  But isn't
package B kind of a client of A in this scenario or does another rule 
apply here?  NOTE:  I realize I might be using the word "client"
inappropriately here.  I don't know the right terminology, so I hope
at least the intent of my question is clear enough.

Using GNAT GPL 2016 for compilation if it makes a difference

Re: Package formal parameter visibility

[Robert A Duff]

Jere <jhb.chat@gmail.com> writes:

> If I use A as a formal parameter into another package, I suddenly get
> visibility of A's formal parameters in that new package.

Yes, formal packages are treated differently.  If you want to know the
exact rules, and why, read AARM-12.7(10-10.b).

- Bob

Re: Package formal parameter visibility

[Jere]

On Sunday, March 26, 2017 at 5:47:51 PM UTC-4, Robert A Duff wrote:
> Jere writes:
> 
> > If I use A as a formal parameter into another package, I suddenly get
> > visibility of A's formal parameters in that new package.
> 
> Yes, formal packages are treated differently.  If you want to know the
> exact rules, and why, read AARM-12.7(10-10.b).
> 
> - Bob

Gotcha!  Thank you for the section reference!