[External] : Re: Diamond in type patterns (was: Reviewing feedback on patterns in switch)

[forax at univ-mlv.fr]

> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Sent: Wednesday, January 26, 2022 1:23:04 PM
> Subject: Re: [External] : Re: Diamond in type patterns (was: Reviewing feedback
> on patterns in switch)

>> The questions we did not answer the last time we talk about that subject
>> - why should we allow raw types here ?

> We already have a clear precedent with type patterns in instanceof — which is
> not a preview feature any more. So for one, now you’re talking about making a
> *change* to the existing language semantics. There are other concerns too.

>> - given that this is equivalent to an instanceof + cast, why we can not use
>> diamond inference on cast ?

> You’re not being clear about what you’re saying, you could be saying either of
> the following (or others):

> - You’re proposing diamond here, but not there, then your proposal is
> inconsistent, and therefore stupid.
> - I love your proposal, but I think we should additionally talk about other
> places to use diamond as well.

> I can’t tell which of these you’re saying, or maybe its something else?

I think we should figure out how it should work on cast and then we can happily applied it on patterns. 

>> - how this inference work ? Is is the same inference than with the diamond
>> constructor ?

> Again, I can’t tell whether you’re saying “this is dumb, it can’t work”, or
> “this is great, but I can’t figure out the details.”

The rules for what you can use as argument of a type parameter when doing a new and when doing a cast are not the same, 
for examples, 
new ArrayList<?>() is not a valid code while (ArrayList<?>) is a perfect valid cast, 
new ArrayList<String>[3] is not a valid code while (ArrayList<String>[]) may or may not be a valid cast, 
new ArrayList<Object>() is a valid code while (ArrayList<Object>) may or may not be a valid cast . 

despite the syntax being the same, the diamond syntax, i don't think we can reuse the same inference rules between the new diamond and the cast diamond. 

regards, 
Rémi 
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