VHDL Issue Number:      0249
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Visibilty outside of design units is not well-defined.
Related Issues          92, 110, 111, 113, 242, 245, tangential 35, 243, possible
48
Relevant LRM Sections:  
Key Words and Phrases:  library visibility, declarative region, scope
Current Status:         Submitted 
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1050
-----------------------
Date Submitted:         1992/01/10
Author of Submission:   Alex Zamfirescu
Author's Affiliation:   Vantage Analysis Systems Inc.
Author's Post Address:  42848 Christy Street, Suite 200
                        Fremont, CA. 94538  
Author's Phone Number:  (510) 659-0901
Author's Fax Number:    
Author's Net Address:   uunet!vntage!anz
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/07/27 00:25:39 $


Description of Problem
----------------------
Consider the folowing example:

   library L;                 --Line 1
   entity E is                --Line 2
                              --Line 3 **
   end E;                     --Line 4
                              --Line 5
   use L.p.all;               --Line 6
   architecture A of E is     --Line 7
                              --Line 8  **_continued
   begin                      --Line 9  **_continued
                              --Line 10 **_continued
   end A;                     --Line 11

The LRM is not clear about the visibility of L outside the architecture 
(line 6). The scope of L starts after the ";" on line 1 and ends at the end of 
the declarative region associated with the entity (in our example line 11). 
The declarative region for the entity e is marked by "**" and "**_continued"
for the continuation inside the architecture outer block. This declarative
region does not include line 6 in our example. However, according to the 
majority interpretation, the use clause on line 6 is legal. It is not clear 
how the scope of L gets out of the declarative region for e-a and then back in.


Proposed Resolution
-------------------
Assume an "outer declarative region" that has the special properties like
the following: 

1. Library logical names and the name of the design unit are considered to be
declared in this "outer or global declarative region."

2. If a string of design files is analyzed in order, the "outer declarative
region" starts at the top of the first analyzed design file and continues
into the next analyzed design file. 

3. The second and subsequent appearances of the same library logical name
in the same library clause or in diferent library clauses defining libraries
for the same design unit have no effect and should be ignored.

4. An architecture body identifier may be the same as the identifier for
the corresponding entity declaration. 

5. Other special rules as required. 


VASG-ISAC Analysis & Rationale
------------------------------ 
Missing or purely defined semantic for visibility outside design units is  
apparently leading to similar problems as described in the following issue
reports:

IR-0092: Visibility of an architecture body identifier is unclear.

IR-0110: May library names be redefined between the analysis
         of a primary unit and a corresponding secondary unit?

IR-0111: What are the semantics of duplicate library name definitions?

IR-0113: Scope of expanded names is unclear.

IR-0242: Is every library unit assumed to occur within package
                 STANDARD ?

IR-0245: Direct visibility rules for USE clauses are incomplete  

The VASG-ISAC Recommendation  for this IR might effect the correct 
resolutions for the following reports:

IR-0035: Enumeration literals, physical units, and library names
         cannot be attributed.

IR-0048: Entity aspect in default binding indication is unclear.

IR-0243: Scope and visibility rules for predefined attributes.


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


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