[Issue 2095] covariance w/o typechecks = bugs

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=2095



--- Comment #19 from Bruno Medeiros <bdom.pub+deebugz at gmail.com> 2010-11-18 11:10:49 PST ---
I've looked at Stewart Gordon's proposal, and I agree that they are safe and
sound (although it may need to be more detailed or cleaned-up a bit). I
actually had prepared a post over a month ago detailing what is basically the
same proposal and underlying conclusions as Stewart's proposal. I did it when I
came across the code sample in bug #2544, but before I read Stewart's proposal
(which I read only recently). I didn't actually post the text I prepared yet,
since I was waiting to clear up my backlog of D newsgroups message. :S
In any case, the conclusions are the same, especially to the point of realizing
the connection to Java's wildcard generics (which, BTW, are the only way to
express this use case safely, but without further loss of type system
functionality).

As a simple solution, I recommend we adopt Stewart's proposal, which is good
enough I think.
The very best solution would be to have a concept like Java's wildcard's, but
that is too complex in implementation to consider any time soon.

@bearophile:
"Some runtime data info may be added, then. There is already some of it for
classes and modules."

Are you out of your mind? Classes are not like arrays and pointers. These are
supposed to be lightweight data types, it's out of place for D to have that
extra runtime data in these lightweight data types (arrays and pointers). It
worries me that you suggested this change without even considering an approach
based on fixing/improving the (static) type system.

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