VHDL Issue Number:      2020

Language_Version:       VHDL-2002
Classification:         Language Definition Problem
Summary:                keyword REPORT is over-used
Relevant_LRM_Sections:  8.2 Assertion statement 8.3 Report statement
Related_Issues:         Changes to the LRM such as making a final ; 
                        optional in port and generic declarations
                        makes me nervous.  I'm afraid of unexpected
                        consequences.
Key_Words_and_Phrases:  assert, report, ;
Authors_Name:           Paul Butler
Authors_Phone_Number:   512 683 8743
Authors_Fax_Number:
Authors_Email_Address:  Paul.Butler@ni.com
Authors_Affiliation:    National Instruments
Authors_Address1:       11500 N. MoPac
Authors_Address2:       Austin, TX 78759
Authors_Address3:

Current Status:         VASG-Approved

Superseded By:

------------------------
Date Submitted:         20 February 2003
Date Analyzed:          1-Nov-04
Author of Analysis:     Peter Ashenden
Revision Number:        4
Date Last Revised:      11 November 2005

Description of Problem
----------------------

In 1076-1987, the following VHDL would not compile.  In 1076-1993, it
produces unexpected results:

     assert true;
     report "failure detected"
     severity error;

     Because report is both an optional word in an assertion statement
and the first word in a report statement, the above code must compile
with or without the semicolon following the assertion expression.
Extra semicolons, especially at a natural end-of-line, are a common
typing mistake.



Proposed Resolution
-------------------
 
While the report statement is handy, it enables a simple typo like the
one above to wreak debugging havoc.  The typical complaint from local
engineers starts something like, "Modelsim claims that 5 < 2!" when,
in fact, the writer has accidentally created an assertion that passes
followed by a report statement!

In the future, please watch out for LRM enhancements that enable typos
to compile without warning but result in undesirable behavior.


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

The submitter is correct in identifying poor syntax design.  However,
to change the syntax to avoid the problem would not be backward
compatible.  Hence, we must live with the poor design for the report
statement.

Reviewers of future changes should be aware of the importance of good
syntax design.  Issues such as this can be avoided by more extensive
prototyping and validation of proposed language changes before they
are set in stone in a revision of the LRM.  Where resources permit,
such validation should be done.


VASG-ISAC Recommendation for IEEE Std 1076-2002
-----------------------------------------------

No change.


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

Add a note to clause 8.2 Assertion Statement which reads something like:

Note: If a semicolon is inadvertently placed between the assert
condition and the REPORT keyword in an assertion statement then no
error will be detected. Instead, the intended assertion will be
interpreted as an assertion statement with no report expression or
severity expression, followed by a report statement.


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