Simplifying switch labels

[Remi Forax]

----- Original Message -----
> From: "Brian Goetz" <brian.goetz at oracle.com>
> To: "daniel smith" <daniel.smith at oracle.com>, "amber-spec-experts" <amber-spec-experts at openjdk.java.net>
> Sent: Thursday, June 2, 2022 8:08:14 PM
> Subject: Re: Simplifying switch labels

>> In this framing, the restrictions about sets of elements in a single label don't
>> apply, because we're talking about two different labels. But we have rules to
>> prevent various abuses. Examples:
>>
>> case 23: case Pattern: // illegal before and now, due to fallthrough Pattern
>> rule
> 
> Ideally, the fallthrough rule should be about _bindings_, not
> _patterns_.  If P an Q are patterns with no binding variables, then it
> should be OK to say:
> 
>     case P:
>     case Q:
> 
> The rule about fallthrough is to prevent falling into code where the
> bindings are not DA.
> 
>> Note that the second kind of Pattern SwitchLabel is especially weird—it binds
>> 'null' to a pattern variable, and requires the pattern to be a (possibly
>> parenthesized) type pattern. So it's nice to break it out as its own syntactic
>> case. I'd also suggest rethinking whether "case null," is the right way to
>> express the two kinds of nullable SwitchLabels, but anyway now it's really
>> clear that they are special.
> 
> This is a painful compromise.  While this may be a transitional
> phenomena, the rule "switch matches null only when `null` appears in a
> case" is a helpful way to transition people away from "switch always
> throws on null."

It also allows to express that a switch statement on an enum should be exhaustive by adding
  case null -> throw null;

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