VHDL Issue Number:      0239
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Can a resolution function return an unconstrained
                        array?
Related Issues:
Relevant LRM Sections:  2.4, 12.6.1
Key Words and Phrases:  resolution function, unconstrained array
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  
Superseded By:          1062
-----------------------
Date Submitted:         1991/03/22
Author of Submission:   Chuck Swart
Author's Affiliation:   Mentor Graphics Corporation
Author's Post Address:  8500 S.W. Creekside Place
                        Beaverton, OR 97005-7191
Author's Phone Number:  (503) 626-7000
Author's Fax Number:    
Author's Net Address:   cswart@mentor.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.9 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
Can a resolution function return an unconstrained array? For example, is the
following legal VHDL code?

        entity E is
        end E;

        architecture A of E is
          type T is array (natural range <>) of Bit_Vector (1 to 5);
          function F (A: T) return Bit_Vector is
          begin
            return A(A'left);
          end F;
          signal S : F Bit_Vector (1 to 5);
          signal S1: F Bit_Vector (1 to 10);
        begin
        end A;

In section 2.4 the LRM requires that "The type of the return value of the
function must also be that of the signal."  It does not require that the
subtypes match.

Section 12.6.1 states:

        "For a scalar signal S, both the driving and effective values
        must belong to the subtype of the signal.  For a composite
        signal R, an implicit subtype conversion is performed to the
        subtype of R; for each element of R, there must be a matching
        element in both the driving and the resolved value, and vice
        versa."

It seems that the above code is technically legal.  However, allowing
unconstrained parameters as return types to resolution functions gives the user
no real benefits and prevents tools from early error detection.

If unconstrained arrays are allowed then the test required by section 12.6.1
can only be performed at simulation time.  If the return value is required to
be constrained then the test (for arrays) can always be performed at
elaboration time and can often be done during analysis.


Proposed Resolution
-------------------
Require that the declared return value of the function have a subtype which is
convertible to that of the signal.  It is unreasonable that the return value
have the same subtype as the signal since that requirement could force the use
of different resolution functions for busses of the same type and size but with
different index ranges.


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


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


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