[fpc-devel] Minor debate with ISO standard on case blocks

[Michael Van Canneyt]

On Mon, 29 Jul 2019, J. Gareth Moreton wrote:

> Hi everyone,
>
> So there's been an issue raised 
> <https://bugs.freepascal.org/view.php?id=35905> in regards to ISO compliance 
> with case blocks (when under $mode iso).  Currently, a compiler error is 
> raised if a case block does not have exhaustive coverage for the input type 
> (it's a warning on other modes).
>
> According to ISO7185 <http://pascal-central.com/docs/iso7185.pdf>, page 55, 
> section 6.8.3.5:
>
> "On execution of the case-statement the case-index shall be evaluated. That 
> value shall then specify execution of the statement of the case-list-element 
> closest-containing thecase-constant denoting that value. One of the 
> case-constants shall be equal to the value of the case-index upon entry to 
> the case-statement; otherwise, it shall be an error."
>
> Given it says "on execution", it implies this all occurs at run-time, 
> especially as 'case-index' is usually a variable or otherwise 
> non-deterministic at compile-time, hence the error should be a run-time 
> error, not a compile-time error.  I think the issue is that FPC won't compile 
> some well-known ISO Pascal code 
> <https://bugs.freepascal.org/view.php?id=35859> because some of the case 
> blocks don't exhaustively cover all inputs (which itself might be a sign of 
> bad programming, but could be justified if said values are determined to be 
> logically impossible, in which case the else blocks should contain assertions 
> or internal errors).
>
> Just interpreting the standard, I think that the error should be run-time, 
> not compile-time (although definitely keep the warning).  That's just my take 
> on it though.  If it is to be changed, it should be simple enough by 
> configuring the 'elselabel' field to point to an error-raising routine rather 
> than 'endlabel' (which occurs if there's no else block).

I tend to agree with your reading of the spec.

Michael.