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Date: Fri, 18 Oct 2013 09:01:46 -0700 (PDT)
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Subject: Possible GNAT problem with aliased parameters.
From: Shark8 <onewingedshark@gmail.com>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:183658
Date: 2013-10-18T09:01:46-07:00
List-Id: <comp.lang.ada>

The Ada 2012 Rational says the following about Aliased parameters:

>    The other change in Ada 2012 concerning parameters is that they may be
>    explicitly marked aliased thus procedure P(X: aliased in out T; ... );
>    As a consequence within P we can write X'Access.

Yet, when such parameters are thus used (X'Access) the compiler generates the following message "access-to-variable designates constant" and fails the compilation.

-- Example:
-----------

    Package Test is
        Type Maze(<>) is tagged private;
        
        None : Aliased Constant Maze;
        
        Function Create( North, South, East, West : Aliased Maze'Class:= None )
          return Maze;
        
        
    Private
        
        Type Paths is Array(Positive range <>) of Access Maze'Class;
          
        Type Maze( Length : Natural ) is tagged record
            Exits : Paths(1..Length);
        end record;
        
        -- Note: Expression-function used so everything is defined in-spec.
        Function Create( North, South, East, West : Aliased Maze'Class:= None )
          return Maze is
          ( Maze'(Length => 4, Exits => (North'Access, South'Access, East'Access, West'Access)) );
          
          None : Aliased Constant Maze := ( Length => 0, Others => <> );
        
    End Test;