[fpc-devel] [Suggestion] Enumeration range-check intrinsic

[Ondrej Pokorny]

On 13.07.2019 14:32, Michael Van Canneyt wrote:
> I yesterday checked a database project I have. I counted 25 enumerateds
> which are stored in the database. Without fail, for each type there is 
> somewhere code in a type helper:
>
> if (MyDatabaseValue>=0) and (MyDatabaseValue<=Ord(High(TMyENum))) then
>   MyENumValue:=TEnum(MyDatabaseValue))
> else
>   EConvertError.Create('Invalid value for TMyEnum : %d',[MyDatabaseValue])

I do exactly the same - check the low/high bounds in a type helper :) 
Yes, and I am tired of typing it as well :)


> That the case where you would write
>
> if MyEnumValue is TMyEnumValue then
>
> can 'by default' be optimized away because as far as the compiler is
> concerned this is a tautology, is for me a perfectly acceptable and
> defendable point of view.

I agree.


> Even to such a degree that the compiler would
> simply reject your code by not allowing is/at on a variable of the type
> itself. Also defendable.

No - not this one. Don't forget about e.g. generics:
function Test<T>(MyValue: T): Boolean;
begin
   Result := MyValue is TMyEnum;
end;
would compile for all enumeration types but not for MyValue:TMyEnum. 
That doesn't make sense.

Better to have a compiler hint/warning when such a construct is found 
and optimized. This perfectly corresponds with the fact that also 
constructs like "SmallIntValue is Integer" can be optimized away but 
must be allowed.

Ondrej