VHDL Issue Number:      0162
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Qualified expression checks may be too restrictive.
Related Issues:         Superseded By IR 1034 (Andy Tsay)
Relevant LRM Sections:  7.3.4.
Key Words and Phrases:  Qualified expressions, subtype checks
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1034
-----------------------
Date Submitted:         1991/03/14
Author of Submission:   Paul Menchini
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  (919) 361-1913
Author's Fax Number:    
Author's Net Address:   mench@clsi.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/08/04 01:43:10 $


Description of Problem
----------------------
Consider the opening sentence of Section 7.3.4 (on page 7-12):

        A qualified expression is used to explicitly state the type, and
        possibly the subtype, of an operand that is an expression or an
        aggregate.

Also consider the final sentence of the same section:

        The evaluation of a qualified expression evaluates the operand
        and checks that its value belongs to the subtype denoted by the
        type mark.

The first sentence quoted implies that subtype checks are not performed if the
type mark denotes a type; however, the second sentence quoted seems to require
subtype checks regardless.

These checks are not problematic for scalar types, since every scalar type is,
in effect, its own subtype.  There may be problems with regard to array types,
however.

Consider the case of the type mark denoting a (constrained) array subtype.  The
subtype check seems to imply that the subtype of the expression or aggregate
must *match* that of the array subtype; in other words, an implicit subtype
conversion is not permitted.  This seems overly restrictive.

Finally consider the case of the type mark denoting an (unconstrained) array
type.  The second sentence still seems to require a subtype check, although
there is no subtype to check.

Note that Ada also suffers from these same problems.  (To my knowledge, no Ada
Issues have been recorded regarding qualified expressions.)  It seems that the
array case was overlooked.


Proposed Resolution
-------------------
Modify the final sentence of Section 7.3.4 to consider scalar and array types
separately.  The scalar case can be handled as is.  The array case should
consider array types and subtypes separately: array types should not require a
subtype check, and array subtypes should permit an implicit subtype conversion
to be performed.


VASG-ISAC Analysis & Rationale
------------------------------
TBD


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
TBD


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
TBD