VHDL Issue Number:      0192
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                VPI Issue 16 -- Redefining predefined attributes
Related Issues:         TBD
Relevant LRM Sections:  TBD
Key Words and Phrases:  TBD
Current Status:         ISAC-Approved
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  Section 5.1 has been updated as suggested.
Superseded By:          N/A
-----------------------
Date Submitted:     1991/03/15
Author of Submission:   J. R. Armstrong
Author's Affiliation:   Virginia Polytechnic Institute and State University
Author's Post Address:  Bradley Department of Electrical Engineering
            Virginia Polytechnic Institute and State University
            Blacksburg, VA 24061
Author's Phone Number:  (703) 231-4723
Author's Fax Number:    
Author's Net Address:   JRA@VTVM1.CC.VT.EDU
-----------------------
Date Analyzed:          1991/12/04
Author of Analysis:     Clive Charlwood
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
  Can a predefined attribute be redefined?

  Test File: ch05/02010102.vhd

  The predefined attribute left is redefined in the test.  The
  LRM  doesn't  describe whether a predefined attribute can be
  redefined or not.


  -- ##50002112248FF5100111************************************************
  --
  --+ File: [SUITE.CH05]02010102.vhd
  --
  --+ Copyright: (c) 1989 by Virginia Tech. All rights reserved.
  --
  --+ LRM_Version: IEEE Std. 1076-1987
  --
  --+ Subsets: All
  --
  --+ Description:
  --
  --+ Test_point - (050000_020101) "A specification always relates to entities
  --  that already exist."
  --
  --+ Test_objective (2) -  Test that no error is reported when specifications
  --  relate to entities that already exist.
  --
  --+ References: Secondary LRM Coverage: 050100_010101
  --
  --+ Instructions: None
  --
  --+ Keywords: specification, entities
  --
  --+ Test_Results: Expect_Success. Output_Checked.
  --
  --+ Test_Type: Locally Static Semantic
  --
  --+ History: Created KSC/VTech 4-16-90
  --
  --+ Comments: None
  --
 
--****************************************************************************
  package P1 is
          type a is range 1 to 10;
          attribute left: integer;
          attribute left of a : type is 5;   --- No_failure_here

  end P1;


Proposed Resolution
-------------------
TBD


VASG-ISAC Analysis & Rationale
------------------------------
There are two questions that need to be answered.

1) Is it legal to attribute an entity if a predefined attribute of the
   same name is already defined for it? E.G. Can you specify a user defined
   attribute LEFT for a type?

2) Is it legal to attribute an entity with a user defined attribute with 
   the same (simple) name as a predefined attribute even though the predefined
   attribute is not  defined for said entity? E.G. Can you specify a user
   defined attribute LEFT for a constant?

The LRM is clear on the first points. Section 5.1 (just above the example)
states that:

    It is an error if a given attribute is associated more than once
    with a given entity. Similarly IT IS AN ERROR IF TWO DIFFERENT 
    ATTRIBUTES WITH THE SAME SIMPLE NAME ARE BOTH ASSOCIATED WITH A
    GIVEN ENTITY.

Since the predefined attributes are implicitly declared when the entity is
declared. It is an error to attribute the entity a second time.

The second point is not explicitly covered by the LRM. We can see no
reason why it should not be legal.

In conclusion:
    ...
             type a is range 1 to 10;
                 constant c: integer := 0;

                 attribute left: integer;

                 attribute left of a : subtype is 5;    -- Error
                 attribute left of c : constant is 5;   -- No Error
        ...


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
No changes are needed.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
For clarity, the above paragraph should be amended to explicitly include 
predefined attributes.