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Date: Sat, 20 Oct 2012 17:37:36 -0700 (PDT)
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Subject: Is this a bug in Ada.Numerics?
From: Leo Brewin <leo.brewin@internode.on.net>
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Date: 2012-10-20T17:37:36-07:00
List-Id: <comp.lang.ada>

In the course of my dabblings in Ada I've come across a curious "behaviour" that seems to me, with my limited knowledge of Ada, to be a bug.

Here is a simple package spec

   with Ada.Numerics.Generic_Complex_Types;
   with Ada.Numerics.Generic_Complex_Elementary_Functions;

   package foo is

      type Real is digits 18;
		
      package Complex_Types is new
         Ada.Numerics.Generic_Complex_Types (Real);

      use foo.Complex_Types;
      -- subtype Complex is foo.Complex_Types.Complex;
      
      procedure bar (z : in out Complex);
   
   end foo;

and the package body

   package body foo is

      package Complex_Maths is new
         Ada.Numerics.Generic_Complex_Elementary_Functions (Complex_Types);
   
      procedure bar (z : in out Complex) is
      begin
         z := Complex'(Re(z),0.0);
      end bar;

   end foo;

As it stands this package compiles under GNAT GPL 2012. But if the "subtype" declaration in the package spec is un-commented then the compile fails with the following errors

04: instantiation error at a-ngcefu.ads:24
04: "Complex" is not visible (more references follow)
04: instantiation error at a-ngcefu.ads:24
04: non-visible declaration at foo.ads:11
04: instantiation error at a-ngcefu.ads:24
04: non-visible declaration at a-ngcoty.ads:42, instance at foo.ads:8
04: instantiation error at a-ngcefu.ads:24
04: non-visible declaration at a-ngcoty.ads:42, instance at a-ngcefu.ads:18

Is this behaviour correct? My limited understanding is that the "use foo.Complex_Types" should have made visible all types and operations on "Complex" and thus the extra "subtype" should be redundant and should not cause an error.

I've tried a few variations on the above and I found that if I
1) Comment out the "Package Complex_Maths" declaration, OR
2) Create a single procedure from the above package spec/body
then the code compiles happily with or without the "subtype" declaration.

If anybody could explain this behaviour to me I would be very grateful.

Cheers,
Leo