VHDL Issue Number:      1086
Classification:         Language Definition Problem
Language Version:       VHDL-93
Summary:                Use of character literal or operator symbol in alias
Related Issues:         ?
Relevant LRM Sections:  4.3.4, 2.3.1 
Key Words and Phrases:  alias, operator symbol, character literal, overloading
Current Status:         ISAC-Approved
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1994/03/22
Author of Submission:   Anne Robert
Author's Affiliation:   LEDA S.A.
Author's Post Address:  35 Av du Granier 38240 MEYLAN FRANCE
Author's Phone Number:  (+33) 76 41 92 43
Author's Fax Number:    (+33) 76 41 92 44
Author's Net Address:   anne@leda.fr
-----------------------
Date Analyzed:          1997/06/25
Author of Analysis:     Dale Martin
                        Peter Ashenden
Revision Number:        $Revision: 1.1 $
Date Last Revised:      $Date: 1995/07/25 18:37:19 $


Description of Problem
----------------------
No restrictions have been made on the use of a character literal or operator
symbol as the designator of an alias. For example, according to the LRM, it is
legal to use a character literal to alias an object or a function.


Proposed Resolution
-------------------
+ A character literal can be used to alias the designator of an enumeration
literal only.

+ An operator symbol can be used only as the alias designator of a function
whose parameter and result type profile match operator profile requirements.



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

As the issue author notes, the syntax rule for an alias declaration
allows the alias designator to be an operator symbol or a character
literal.  The VHDL-93 LRM does not explicitly restrict the class of
object or entity that may be aliased with an operator symbol or a
character literal.

There are some implicit restrictions that arise from the syntactic
contexts in which an operator symbol or a character literal can
otherwise be written.  For example, a character literal can not occur
as a type_mark, so aliasing a type name with a character literal is
useless.  Similarly, aliasing a binary operator symbol to a function
with only one parameter would not be sufficient to allow unary use of
the operator.

The resolution proposed by the issue author is not sufficient to
prevent significant obfuscation in a model.  For example, the
following alias:

        alias 'a' is std.standard.'b' [ return std.standard.character ];

would still be legal under the proposed resolution, and would make the
expression

        'a' = 'b'

have the value true!

However, the proposed resolution makes the intention of operator
symbol and enumeration literal alias designators clear, and is thus
desirable.  It would, for example, make the following illegal:

        constant c : integer := 1;
        alias 'c' is c;

which would otherwise make the expression

        'c' = 1

have the value true.


VASG-ISAC Recommendation for IEEE Std 1076-1993 
----------------------------------------------- 

If the alias designator is a character literal, the name must
denote an enumeration literal.  If the alias designator is an
operator symbol, the name must denote a function, and that
function then overloads the operator symbol.  The operator
symbol and the function must meet the requirements of 2.3.1.


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

In 4.3.3, add the following paragraph after line 549:

        If the alias designator is a character literal, the name must
        denote an enumeration literal.  If the alias designator is an
        operator symbol, the name must denote a function, and that
        function then overloads the operator symbol.  The operator
        symbol and the function must meet the requirements of 2.3.1.