VHDL Issue Number:      0068
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Bad definition for LeftOf and RightOf attributes at
                        the end cases.
Related Issues:         
Relevant LRM Sections:  14.1
Key Words and Phrases:  LeftOf, RightOf
Current Status:         ISAC-Approved
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  LRM was updated.
Superseded By:          N/A
-----------------------
Date Submitted:         1989/02/10
Author of Submission:   Doug Dunlop
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:    
Author's Net Address:   dunlop@inmet.inmet.com
-----------------------
Date Analyzed:          1990/09/27
Author of Analysis:     Alex Zamfirescu
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/08/03 16:27:52 $


Description of Problem
----------------------
For the subtype
           subtype T is Natural range 10 downto 1;
It is not clear what is the value of T'LeftOf(10).  The only possibility, 11,
does not make sense in light of the fact that in NO subtype (Natural or T) is
11 to the left of 10.  It is clear that the line that reads "an error occurs if
X equals T'BASE'LEFT" should read "an error occurs if X equals T'LEFT".


Proposed Resolution
-------------------
Assume the above correction and the corresponding one for RightOf.


VASG-ISAC Analysis & Rationale
------------------------------
Since the result value of T'LEFTOF(X) is by LRM Chapter 14 "the value which is
to the left of the parameter in the range of T", the restriction "an error
occurs if X equals T'BASE'LEFT" is too soft. The intent of the language 
designers was to restrict the return value of the function T'LEFTOF(X) from 
falling out of the range of T.
Therefore the restriction for T'LEFTOF(X) should be;

An error occurs if X equals T'LEFT, or if X is not in the range T'LOW to
T'HIGH.
 
The corresponding problem analysis applied to the attribute RIGHTOF leads to 
the similar restriction;
  
An error occurs if X equals T'RIGHT, or if X is not in the range T'LOW to
T'HIGH.    


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
Change the Restrictions field for the predefined attributes T'LEFTOF(X) 
LRM chapter 14 from 
       An error occurs if X equals T'BASE'LEFT.
to 
       An error occurs if X equals T'LEFT, or if X is not in the range
       T'LOW to T'HIGH.
 
Change the Restrictions field for the predefined attributes T'RIGHTOF(X) 
LRM chapter 14 from 
       An error occurs if X equals T'BASE'RIGHT.
to 
       An error occurs if X equals T'RIGHT, or if X is not in the range
       T'LOW to T'HIGH. 


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Another posibility would be to define these attributes so that 
T'LEFTOF(T'LEFT) = T'RIGHT and T'RIGHTOF(T'RIGHT) = T'LEFT and 
the error occuring only when X is not in the range T'LOW to T'HIGH.