VHDL Issue Number:      1073
Classification:         Language Definition Problem
Language Version:       VHDL-93
Summary:                The meaning of a qualified expression is in dispute
Related Issues:         0244
Relevant LRM Sections:  7.3.4
Key Words and Phrases:  Qualified expressions, matching elements
Current Status:         ISAC-Approved
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1991/09/19
Author of Submission:   Doug Dunlop (edited by Bill Paulsen)
Author's Affiliation:   Intermetrics, Inc.
Author's Post Address:  4733 Bethesda Ave, #415
                        Bethesda, MD  20814
Author's Phone Number:  (301) 657-3775
Author's Fax Number:    N/A
Author's Net Address:   dunlop@inmet.inmet.com
-----------------------
Date Analyzed:          1995/04/10
Author of Analysis:     Clive R. Charlwood
Revision Number:        $Revision: 1.8 $
Date Last Revised:      $Date: 1995/05/15 21:26:37 $


Description of Problem
----------------------
(This IR supersedes IR 0244.)

A customer has explained to me that Valid and CLSI give different meanings
to certain qualified expressions.

In particular, consider the case of a qualified expression
      type_mark'(expression)
where type_mark is a constrained array subtype.  Furthermore, suppose the
direction of this subtype differs from that of the expression but
nevertheless there are "matching elements" between the 
subtype and the value of the expression.

In this case should the evaluation of the qualified expression result in
a run-time error because the value of the expression does not belong "to
the subtype denoted by the type mark"?


Proposed Resolution
-------------------
The direction mismatch in this case should cause the subtype check required
in 7.3.4 to fail.


VASG-ISAC Analysis & Rationale
------------------------------
1) A qualified expression is not intended to CONVERT an operand to
   the specified type or subtype. It is used to explicitly state the type,
   and possibly the subtype, of the operand.

2) A qualified expression may COERCE the operand by providing the
   context required to determine its type and subtype.

Therefore if an expression has an explicit subtype then this subtype 
must be compatible with the specified subtype. This includes that 
bounds and DIRECTION.

The ISAC felt that the phrase

         "The evaluation of a qualified expression evaluates the operand
          and checks that its value belongs to the subtype denoted by the
          type mark" (Page 105, line 485)

covers this point completely, and that no changes to the LRM are required
to resolve this issue.

VASG-ISAC Recommendation for IEEE Std 1076-1993
-----------------------------------------------
No changes are required.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
No changes are required. However, the LRM author may want to consider
making this point more explicit.