Sorry Remi, my original email was a little cryptic! What Brian writes is what I
propose to spec, and what I was asking for opinions on. I log your approval :-)
Gavin
> On 20 Sep 2019, at 15:21, fo...@univ-mlv.fr wrote:
>
>
>
> De: "Brian Goetz" <brian.go...@oracle.com>
> À: "Remi Forax" <fo...@univ-mlv.fr>
> Cc: "Gavin Bierman" <gavin.bier...@oracle.com>, "amber-spec-experts"
> <amber-spec-experts@openjdk.java.net>
> Envoyé: Vendredi 20 Septembre 2019 16:12:48
> Objet: Re: Draft JLS spec for JEP 305: Pattern matching for instanceof
> 1. The instanceof operator restricts the type to be a reifiable reference
> type. The spec currently keeps that restriction for type test patterns too.
> But should we go further, i.e. will people expect to be able to say the
> following (given that this *declares* a pattern variable l)?
>
> if (o instanceof List<Integer> l) {
> …
> }
>
> If we allow such case, the compiler will have to emit an unchecked warning
> because this code is not safe.
>
> This is not correct. If you look at what I wrote in
>
> http://cr.openjdk.java.net/~briangoetz/amber/pattern-semantics.html
> <http://cr.openjdk.java.net/~briangoetz/amber/pattern-semantics.html>
>
> it is possible to do this in a safe manner, by appealing to whether the cast
> conversion is checked or not:
>
> Generic type patterns are permitted (this is a relaxation of the current
> semantics of the instanceof operator, which requires that the type operand is
> reifiable.) However, when determining applicability involves cast
> conversions, the pattern is not applicable if the cast conversion would be
> unchecked. So it is allowable to say
>
> List<Integer> list = ...
> if (list instanceof ArrayList<Integer> a) { ... }
> but not
>
> List<?> list = ...
> if (list instanceof ArrayList<String> a) { ... }
>
> So if we permitted non-reifiable types in type patterns, it would only be
> where a cast conversion would not produced an unchecked warning, and this is
> not unsafe.
>
> yes, you're right.
> need to get some coffee ...
>
> so i'm ok with that construct.
>
> Rémi
>
