VHDL Issue Number:      1048
Classification:         Language Clarification
Language Version:       VHDL-93
Summary:                Deferred constant reference before complete definition.
Related Issues:         0236, 0020
Relevant LRM Sections:  2.6
Key Words and Phrases:  Deferred constant usage
Current Status:         ISAC-Approved
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/11
Author of Submission:   Laurence Groves (edited by Bill Paulsen)
Author's Affiliation:   Synopsys Inc.
Author's Post Address:  1098 Alta Ave.
                        Mountain View, CA 94043
Author's Phone Number:  (415) 962-5426
Author's Fax Number:    N/A
Author's Net Address:   larry@Synopsys.COM
-----------------------
Date Analyzed:          1995/04/10
Author of Analysis:     Clive R. Charlwood
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/06/20 22:56:36 $


Description of Problem
----------------------
(This IR supersedes IR 0236.)

Consider the following code fragment from a package declaration:

   constant C : bit_vector(1 to 3);
   signal   S : bit_vector(C'range);    -- failure here

It appears that the LRM is ambiguous on this point.  LRM 2.6, paragraph 5:

   Within a package declaration that contains the declaration of a
   deferred constant, and within the body of that package (before
   the end of the corresponding full declaration), the use of a name
   that denotes the deferred constant is only allowed in the default
   expression for a local generic, local port, or formal parameter.
   The result of evaluating an expression that references a deferred
   constant before the elaboration of the corresponding full declaration
   is not defined by the language.

The last sentence is key:  "... not defined by the language."  The above
case falls into this rather ambiguous statement.

Additionally, in IR 0020 (status: ISAC approved) supports not relaxing
current rules regarding deferred constants.


Proposed Resolution
-------------------
Modify the last sentence of paragraph 5, section 2.6 to read:

   Any other reference to a deferred constant before elaboration of the
   corresponding full declaration is an error.


VASG-ISAC Analysis & Rationale
------------------------------
A careful reading of the first quoted sentence states that
	"... the use of a name ...  is only allowed in the default
   	 expression for a local generic, local port, or formal parameter."

Therfore the above code is clearly illegal.


VASG-ISAC Recommendation for IEEE Std 1076-1993
-----------------------------------------------
No LRM change is required.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
We recommend that the next version of the language 
consider treating references to a deferred constant differently
from reads. In particular, adding or reading an attribute
should probably be legal as soon as the constant is declared.