Feature request: custom default annotation attributes

[Brian Goetz]

While I'm the first to admit that the design of annotations incorporated 
a great deal of optimism about its sufficiency, I think the 
return-on-complexity for such things is not good enough to warrant 
working on this.

> Now I was thinking: wouldn't it be nice to be able to define a custom 
> default attribute? That could be done using an annotation, let's call 
> it @DefaultAttribute. 

Actually it can't.  The cardinal design principle of annotations is: 
they do not affect language semantics.  So you'd need actual language 
support here (such as marking the default attribute with a keyword like 
"default-attribute".)  That's not impossible, but there's clearly a host 
of conditions and interactions to deal with. Yes, they're mostly 
trivial, but there's always more of them than one imagines at first.  
But if we do this feature (which seems trivial, but no language feature 
is trivial), we're implicitly deciding to delay or not do some other 
feature.  And the other features on the menu all offer a much greater 
return.

(Such features are like single-purpose kitchen appliances; a countertop 
hot-dog cooker might be the best way to make hot dogs, but single-use 
appliances usually offer a pretty poor return on counter space.  (Except 
the rice cooker; that's the one single-purpose appliance we have in our 
house, and I wouldn't give it up.))

So, given an infinite budget for implementation, language surface, and 
complexity, its not something I'd rule out because its terrible 
(clearing this bar is actually pretty good), but realistically I don't 
see it coming near the top of the list of features we'd want to invest in.