VHDL Issue Number:      0250
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Checking the target type of an aggregate signal assignment
Related Issues:         0221
Relevant LRM Sections:  8.3
Key Words and Phrases:  assignment, type checking
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1025
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Date Submitted:         1992/01/01
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: 1995/05/13 21:53:48 $


Description of Problem
----------------------
In the case of an aggregate signal assignment, the LRM is not clear about 
what kind of checks are needed for the aggregate assignment target type, 
what this type is (if any), and how the waveform elements should match. 

Waveform elements subtypes in a signal assignment have to match the target
type, which is unknown when target is an aggregate. Base type matching 
between all waveform elements can be done at analysis time. There are cases 
when subtype checking for the waveform elements has to be postponed.

Consider the following example:   

   (s_bit_1, s_bit_2) <= "00" after 1 ns, 
                          GENERIC_BIT_VECTOR_TYPE'(others => '0' ) after 2 ns,
                          ('1','0') after 3 ns; 

where GENERIC_BIT_VECTOR_TYPE is a type that depends on some generic values. 
It is not clear what kind of elaboration time checks are needed to validate
the waveform in this case. What kind of checks are required at run time ?


Proposed Resolution
-------------------
There are more consistent ways around this dilemma. 

One is to rule that signal aggregate assignment waveform element types 
has to be locally static. 

Another would require a target aggregate qualifier in the case when 
not all waveform element subtypes are locally static. 


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


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


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