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From: "Grein, Christoph" <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Re: Dispatching and generics - language lawyer question
Date: Wed, 24 Jul 2002 07:33:46 +0200 (MET DST)
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Date: 2002-07-24T07:33:46+02:00

> There is a problem with "reemergence" of predefined operators ...

For untagged types, _redefined_ equality does not correctly compose and 
_predefined_ equality reemerges in generics if not properly transferred by a 
formal parameter:

  type X is something;
  function "=" (L, R: X) return Boolean;  -- redefine

  type Y is record
    C: X;
  end record;

  A, B: Y;
  L: Boolean := A = B;  -- here predefined "=" on component C is used, not the
                        -- equality redefined above

  generic
    type T is private;
    --with function "=" (L, R: T) return Boolean is <>;
  package P is ...

  package P_on_X is new P (X);  -- here also predefined "=" on X is used,
                                -- not the equality redefined above

You have to uncomment the generic parameter function to prevent reemergence.

This whole astonishing rule has been introduced because of compatibility with 
Ada 83 (note there were no tagged types in Ada 83 so there reemergence could 
safely be avoided).

In Ada 83, it was not possible to redefine "=" without using a trick (I do not 
remember the exact way to do it, I never did this; you have to use generics with 
the a limited private formal type). When you used this trick to redefine 
equality, also the predefined equality reemerged as in the cases above. Thus for 
compatibility, we have this rule in Ada 95.