[Ada] Convention for operations of generlc formals with unknown discriminants

[Arnaud Charlet <charlet at adacore dot com>]

RM 6.3.1 (8) specifies that the operations inherited by a generic formal
tagged derived type with unknown discriminants have convention Intrinsic.
As a consequence, attribute 'Access cannot be applied to suvh an operation.

This rule also makes it impossible to override an inherited operation of a
type derived from such a formal type (a not uncommon usage), so in fact it
probably needs to be relaxed. The straightforward solution adopted here is to
say that the primitives of a type derived from such a formal type take their
convention from the primitive operation of the ultimate non-generic ancestor.

Compiling no_access.adb in ada 2005 mode  must be rejected with the message:

no_access.adb:22:12: prefix of "Access" attribute cannot be intrinsic

---
procedure No_Access is
  package P is
     type Root is tagged null record;
     procedure Proc (X : Root);
  end P;

  package body P is
     procedure Proc (X : Root) is begin null; end;     
  end P;

  generic
     type Formal (<>) is new P.Root with private;
     Value : Formal;
  package G is
     procedure Rien;
  end G;

  package body G is
     procedure Rien is
     begin
        --  attribute reference is illegal because inherited Proc is intrinsic
        if Proc'access = null then null; end if;
     end;

     Thing : Formal := Value;

  begin
     Proc (Thing);
  end G;

begin
  null;
end;
---

Tested on x86_64-pc-linux-gnu, committed on trunk

2013-01-02  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch3.adb (Derive_Subprogram): Enforce RM 6.3.1 (8):
	if the derived type is a tagged generic formal type with
	unknown discriminants, the inherited operation has convention
	Intrinsic. As such, the 'Access attribute cannot be applied to it.

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