VHDL Issue Number:      0041
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Architecture item visibility in configurations is
                        incorrectly defined.
Related Issues:         
Relevant LRM Sections:  1.3, 10.2, 10.3
Key Words and Phrases:  Configuration declaration, visibility
Current Status:         Analyzed
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  Section 10.3 was revised.
Superseded By:          N/A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:          1991/03/21
Author of Analysis:     Paul Menchini (mench@clsi.com)
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/08/03 00:18:20 $


Description of Problem
----------------------
The intention is for a configuration to have visibility over certain items
declared in architectures.  In the LRM however, this is not covered by either
direct visibility or select visibility.  The LRM extends the "scope" of items
declared in architectures, but this does not make them visible.


Proposed Resolution
-------------------
The intent as implied in the "Note:" in LRM 1.3.1 will be implemented.


VASG-ISAC Analysis & Rationale
------------------------------
VHDL's scope and visibility rules were adopted from Ada.  Substantive changes
were made to the list of declarations with extended scope in paragraph 2 of
Section 10.2; paragraphs 4 and 5 of the same section were added to cover
configuration declarations; the list of declarations visible by selection in
paragraph 4 of Section 10.3 has been modified to account for the differences
between VHDL and Ada; 

(In addition, paragraph 6 of Section 10.3 was deliberately modified from that
of Ada (paragraph 14 of Ada Section 8.3).  VHDL:

        A declaration is said to be *directly visible* within a certain
        part of its immediate scope....

(Ada:

        Where it is not visible by selection, a visible declaration is
        said to be *directly visible*.  A declaration is directly
        visible within a certain part of its immediate scope....

(This alteration causes problems noted in other issue reports.)

Unfortunately, no additions to Section 10.3 corresponding to paragraphs 4 and 5
of Section 10.2 were made; thus it is unclear whether a declaration whose scope
is extended into a configuration declaration is thereby made visible.  This
defect must be repaired.

Moreover, another visibility issue related to configurations must be addressed.
Because of the extension of scope (and presumability of visibility) of
declarations provided by paragraphs 4 and 5 of Section 10.2, the sources of
visibility within configuration declarations are unique.  Although no
declarations can be introduced in a configuration declarative part or in a
subordinate block configuration, use clauses can cause declarations to become
directly visible.  Thus, the meaning of a name must be addressed in two
contexts:

    1.  Those names made directly visible via a use clause.

    2.  Those names whose scope (and visibility) extend into the configuration
        declaration.

The LRM does not address the case when there is an interpretation in both
contexts.

(Note that, as noted by the author, the note at the end of Section 1.3.1 is the
only place in the LRM that addresses this issue.)

Association lists within configuration declarations are particularly complex.
Consider a name of the form "F(S)" appearing as a formal in an association
element within a configuration declaration.  The following interpretations must
be considered:

    1.  "F" is the name of a one-dimensional array formal and "S" is an
        expression selecting an element (or perhaps a slice) of the formal.
        "F" is to be found in the context of the names whose scope extends into
        the configuration declaration and "S" is found in the same scope.

    2.  Identical to 1., except that "S" is found in the names made visible by
        the use clauses.

    3.  The union of 1., and 2.  ("S" is found in both contexts.)

    4.  "F" is a type conversion function and "S" is a formal.  "S" is found in
        the context of the names whose scope extends into the configuration
        declaration and "F" is found in the same scope.

    5.  Identical to 3., except that "F" is found in the names made visible by
        the use clauses.

    6.  The union of 4. and 5.

The LRM again does not address how to address this issue.


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


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