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

[Brian Goetz]

OK, I get your point now.  Your concern is not about *inference*, but specifically how *diamond* will snap to the bound when it infers a wildcard, because `new` doesn’t allow wildcards.  But we do something differently for inferring type variables of generic methods (we’ll gladly infer a capture) or locals (we’ll project a capture to a nearby super type without capture.)

On Jan 27, 2022, at 8:20 AM, forax at univ-mlv.fr<mailto:forax at univ-mlv.fr> wrote:



________________________________
From: "Brian Goetz" <brian.goetz at oracle.com<mailto:brian.goetz at oracle.com>>
To: "Remi Forax" <forax at univ-mlv.fr<mailto:forax at univ-mlv.fr>>
Cc: "amber-spec-experts" <amber-spec-experts at openjdk.java.net<mailto:amber-spec-experts at openjdk.java.net>>
Sent: Thursday, January 27, 2022 2:04:35 PM
Subject: Re: [External] : Re: Diamond in type patterns (was: Reviewing feedback on patterns in switch)
It's more an engineering thing here, we have far more casts than switch + pattern in existing code, and given that we suppose (perhaps wrongly) that the semantics of the inference is not exactly one already existing,

I’d like to drill into this supposition.  My supposition (maybe wrong) is that we already solved most of this when we did `var`, with upward projection.

To recap, we spent a lot of time with `var` on what to do about non-denotable types.  These included the null type (banned on the grounds of uselessness), intersection types (allowed), and capture types (sanitized with upward projection.)  The basic idea of upward projection is that when we infer List<cap>, we replace it with a super type that has no capture types, and get List<?> out.  (There’s also a downward projection.)

Let’s start with your examples of where ordinary inference produces an undesirable result, and then evaluate whether either or the projections solves the problem?

I don't think current projections are enough because we may want the inference to insert a wildcard by itsef,
for example with
  Object o = ...
  var list = (List<>) o;

or maybe we should not try to infer such code.

Rémi


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