Is this a bug in generics with default()?

=====================

type
  TEnum = (Unused = '.');
  generic TSomething<T> = class
    function Get: T;
  end;
  TEnumeratedSomething = specialize TSomething<TEnum>;
function TSomething.Get: T;
begin
  // Range check error while evaluating constants (0 must be between 46 and 46)
  exit(Default(T));
end;

Regards,
Ryan Joseph

See https://bugs.freepascal.org/view.php?id=34972 for some comments about Default. So this is a bug in the usage of Default, since Default cannot be guaranteed to return a valid value for a type.

It is even technically impossible to make Default return a valid value for every type due to variant records (you could have overlapping "field1: 1..3;" and "field2: 4..6;").

Jonas

I thought default would just return the first value in the enum but I guess it's just zeros. Maybe we need a better default for objfpc mode.

Regards,
Ryan Joseph

I tried to explain in the rest of my message why that cannot be done. It might be possible in some cases, but that would just create a false sense of security and unpredictable results (because no compile-time range checking is possible for variant records).

Jonas

Ryan Joseph wrote

I thought default would just return the first value in the enum but I guess it's just zeros. Maybe we need a better default for objfpc mode.

The first element is returned by Low(TEnum), and the first value by Ord(Low(TEnum)).

Not even for enums? I get that records should just return zero'd memory but enums seem to be a special case.

Regards,
Ryan Joseph

A record field whose type is an enum should be handled exactly the same as an individual enum.

Jonas