Re: Forcing default representations

[pipex!sunic!news.funet.fi!funic!news.eunet.fi!prime!mits!rkaivola@uunet.u u.net (Risto Kaivola)]

crispen@eight-ball.boeing.com (Bob Crispen) writes:

>And, definitely non-trivially, can someone explain why some compilers
>approve of (and even generate proper code for) the following:

>	type Four_Bytes is range 0..31;
>	...
>	for X use
>         record
>           Y at 0 range Four_Bytes;
>           ...
>        end record;

>and some don't?
> Bob Crispen 

I think that a compiler that accepts this is definitely broken.
According to the relevant chapter (13.4) of the RM, a component
clause is defined as (omitting the italicised prefixes which do
not reflect any syntactic categories, but are there to provide
information to the human reader)

component_clause ::= name AT simple_expression RANGE range;

In 3.5 we find that (again substituting attribute for <range>_attribute)

range ::= attribute | simple_expression .. simple_expression

Actually, according to AI-00498, a range attribute is not allowed
in a component clause, and this fact is reflected in the Annotated
Ada Reference Manual for Ada9X.  But in your example, Four_Bytes
does not even fall into the syntactic category attribute!  I wonder
what the bug causing this behaviour is.  Probably the implementation
simplifies its job by sort-of 'reusing' some of the code it uses in
other situations of this sort, erroneusly.

--
Risto Kaivola
(Internet address:   rkaivola@mits.mdata.fi)