VHDL Issue Number:      0251
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Can expressions using type conversions be locally static?
Related Issues:         
Relevant LRM Sections:  3.1.2, 7.2.5, 7.4
Key Words and Phrases:  locally static, type conversion 
Current Status:         Submitted
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  Section 7.4.1 bullet 9 addresses this IR.
Superseded By:          N/A
-----------------------
Date Submitted:         1992/02/17
Author of Submission:   Chuck Swart
Author's Affiliation:   Mentor Graphics Corporation
Author's Post Address:  8005 S.W. Boeckman Road
                        Wilsonville, OR 97070-7777
Author's Phone Number:  (503) 685-7000
Author's Fax Number:    
Author's Net Address:   chuck_swart@mentorg.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.7 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
    Can an expression which uses a type conversion be locally static?
For example consider the following code fragment (which has been
extracted and simplified from a real model):

ENTITY e IS

    TYPE number_of_bits_in_word_range IS RANGE 8 TO 32;
    CONSTANT number_of_bits_in_word : number_of_bits_in_word_range := 16;

    TYPE word IS RANGE 0 TO 2**integer(number_of_bits_in_word) - 1;

END e;

    Is this legal VHDL? The type conversion is necessary since the right
argument to the exponentiation operator is required to be of type INTEGER.
However, each range bound in an integer type declaration is required to be
locally static, so if type conversions can not be locally static then this is
illegal VHDL code.

    Type conversions (for locally static scalars) are perfectly reasonable
analysis activities, similar in spirit to qualified expressions (although type
conversions between real and integer values also involve changes in
representation.)

    Disallowing type conversions in locally static expressions places an
unnecessary burden on the user.  The only work around in the example would be
for the user to declare an appropriate constant of type INTEGER instead of
using his own integer type.  This restriction violates the intent of providing
user defined integer types.

    At least two vendors currently allow the above example.  At least two
vendors currently disallow the same example.

    Section 7.4 of the LRM deals with locally static expressions.  Type
conversions are not given in the list of locally static primaries.  The
question this IR raises is whether or not this omission was deliberate or
accidental.


Proposed Resolution
-------------------
    Add a sentence to the list of locally static primaries in section 7.4,
similar to the existing one for qualified expressions, something like:

    "9. a type conversion whose type mark denotes a locally static subtype and
        whose operand is a locally static expression;"


VASG-ISAC Analysis & Rationale
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TBD


VASG-ISAC Recommendation for IEEE Std 1076-1987
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TBD


VASG-ISAC Recommendation for Future Revisions
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TBD