!standard 12.05.04 (04)                               96-11-16  AI95-00025/02
!class confirmation 95-06-25
!status WG9 approved 96-12-07
!status ARG Approved 9-0-1  96-10-07
!status work item 95-06-25
!status received 95-06-25
!priority Low
!difficulty Medium
!subject Matching rules for generic formal access-to-constant types

!summary 95-06-25

An access-to-variable type cannot be passed as an actual to a generic
formal access-to-constant type.

!question 95-06-25

RM95-12.5.4(4) states that "If and only if the general_access_modifier
constant applies to the formal, the actual shall be an access-to
constant type".  Is it really intended to forbid an access-to-variable
type from being passed as an actual to a generic formal
access-to-constant type?  (Yes.)

!response 95-06-25

Consider the following example:

    type Actual is access all Integer;
    X: aliased constant Integer;

    generic
	type Formal is access constant Integer;
    package Gp is
        function F return Formal;
    end Gp;

    package body Gp is
        function F return Formal is
        begin
            return X'Access;
        end F;
    end Gp;

    package Ip is new Gp (Formal => Actual); -- Illegal.

If the above were legal, then Ip.F would produce an access-to-variable
value designating a constant, thus allowing a constant to be modified.
This would be very bad.  Hence, the "if and only if" wording of
12.5.4(4).

This should be explained in the AARM.

!appendix 95-06-25

!section 12.5.4(4)
!subject Matching rules for generic formal access-to-constant types
!reference RM95-12.5.4(4);5.95
!from Pascal Leroy
!reference as: 95-5085.a Pascal Leroy 95-1-25>>
!discussion

RM95-12.5.4(4) states that "If and only if the general_access_modifier
constant applies to the formal, the actual shall be an access-to constant
type".

This wording (actually, the "if" part) seems to forbid the following text.  Is
this the intent?  I do not understand why this would cause contract model
problems, or violate the properties of the type.  If there is a hidden reason,
it should be stated in the AARM.

    type Actual is access all Integer;

    generic
	type Formal is access constant Integer;
    package Gp is
    end Gp;

    package Ip is new Gp (Formal => Actual);

_____________________________________________________________________
Pascal Leroy                                    +33.1.30.12.09.68
pleroy@rational.com                             +33.1.30.12.09.66 FAX

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