expression type A is a subtype of pattern type B diagnostics

[Liam Miller-Cushon]

I wondered if that was it: arguably it would be good if `e instanceof Y`
was also an error in this case, but that ship has sailed. There's tension
between trying to use new features to fix mistakes of the past, and having
a smooth onramp to new features, and I know that's been discussed in other
contexts.

The surprising part to me was that the error was introduced, and then
removed as part of the implementation of record patterns.

On Tue, Apr 1, 2025 at 10:04 AM Archie Cobbs <archie.cobbs at gmail.com> wrote:

> On Tue, Apr 1, 2025 at 11:49 AM Liam Miller-Cushon <cushon at google.com>
> wrote:
>
>> So all of this seems deliberate and working as intended, I was just
>> curious what the rationale for the change was. Does anyone remember the
>> history or have pointers to discussions about it on list?
>>
>
> I don't know what the stated rationale was at the time, but the change
> makes intuitive sense to me now: Why should "e instanceof Y x" generate an
> error when "e instanceof Y" does not? That is to say: the correctness of
> the type test itself, which is what the error is about, has nothing to do
> with whether or not you happen to be declaring a new variable that captures
> a successful outcome of that test.
>
> -Archie
>
> --
> Archie L. Cobbs
>
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