Re: Derivation + Access Discriminant = Headaches

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

RM95 3.7(15):

   15  If a discriminant is used in the constraint defining the parent
       subtype, the subtype of the discriminant shall be statically
       compatible (see 4.9.1) with the subtype of the corresponding
       parent discriminant.

AARM comments:

 15.a   Reason:  This ensures that on conversion (or extension via an
        extension aggregate) to a distantly related type, if the
        discriminants satisfy the target type's requirements they
        satisfy all the intermediate types' requirements as well.

 15.b   Ramification:  There is no requirement that the new discriminant
        have the same (or any) default_expression as the parent's
        discriminant.

Thus it's no bug.

However, I don't see why redefined access discriminants shouldn't be allowed
to designate a type derived from (or within the class of) the type
designated by the corresponding parental discriminant.  An extra compiler
check would be required: if an object of type T were to be converted to type
NT (by an extension aggregate), its discriminant, O, would have to be
checked for being in NT'Class (and Constraint_Error, presumably, raised if
not).  Conversions to 'distantly related types' would still only require one
check, however, so the problem mentioned in AARM (15.a) is solved.

Possibly this is something that could be changed at the next revision, if
it's not considered an unnecessary complication (or there's some other
reason why not that I'm missing).

I guess Tuck or Bob will correct me if I'm on the wrong track here (again
;-).

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Nick Roberts
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