VHDL Issue Number:      1021
Classification:         Language Definition Problem
Language Version:       VHDL-93
Summary:                Constrained arrays with negative index bounds are
                        difficult to declare.
Related Issues:         0151
Relevant LRM Sections:  3.2.1.1, 7.3.5, 13.4.1
Key Words and Phrases:  Index subtypes, constrained arrays, implicit conversion
Current Status:         Submitted
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/12
Author of Submission:   Paul Menchini
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  (919) 361-1913
Author's Fax Number:   
Author's Net Address:   mench@clsi.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.6 $
Date Last Revised:      $Date: 1995/05/15 21:26:37 $


Description of Problem
----------------------
Consider the following constrained array declaration:

        type A is array (-1 to 1) of Integer;

This is illegal VHDL!  Consider the second paragraph of Section 3.2.1.1:

        For a discrete range used in a constrained array definition and
        defined by a range, an implicit conversion to the predefined
        type INTEGER is assumed if each bound is either a numeric
        literal or an attribute, and the type of both bounds (prior to
        the implicit conversion) is the type universal_integer.
        Otherwise, both bounds must be of the same discrete type, other
        than universal_integer;  this type must be determined
        independently of the context, but using the fact that the type
        must be discrete and that both bounds must have the same type.

The right bound is clearly of type universal_integer; therefore, so must be the
left bound.  However, this type is explicitly disallowed.  Therefore, to what
type does the index range belong?

There is a further problem.  Assume that the type of the index range can be
determined.  Then the bounds are implicitly converted to this type.  Consider
the last paragraph of Section 7.3.5:

        ....  An implicit conversion of an operand of type
        universal_integer to another integer type, or of an operand of
        type universal_real to another real type, can only be applied if
        the operand is either a numeric literal or an attribute, or if
        the operand is an expression consisting of the division of a
        value of a physical type by a value of the same type; ....

The left bound cannot be implicitly converted according to this rule.

The situation is even worse when user-defined types enter into the picture:

        type MyInt is range -10 to 10;
        type B is array (0 - 1 to 1 + 1) of Integer;

In this example is the (sub)type of the index universal_integer, Integer, or
MyInt?


Proposed Resolution
-------------------
Add LRM language to allow these cases to be legal VHDL and to be unambiguously
interpreted.


VASG-ISAC Analysis & Rationale
------------------------------
TBD


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


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