RFR: 8299444 java.util.Set.copyOf allocates needlessly for empty input collections [v2]

[Rémi Forax]

On Tue, 24 Jan 2023 13:40:37 GMT, Viktor Klang <duke at openjdk.org> wrote:

>> `coll instanceof ListN<?> list` should work.
>
> @forax @stuart-marks Yeah, that works. It's unfortunate that it's not possible to match on the actual (generic) type, as then both sides of the || could use type unification to avoid having to do the cast on the return value as well. Submitted a commit to switch to the wildcard version.

Avoiding the cast to `List<E>` requires two things:
- we need a way to say at declaration that a parameter type E is covariant, this is not true for List but this is true for ImmutableList (so true for List2 and ListN).
- we need a way to do an union between two types when doing pattern matching .

This former is still in discussion as a feature of Valhalla (it's a part of Dan Smith Phd), the later is still in discussion as a feature of Amber, so maybe in the future this will be possible is the star aligned.

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PR: https://git.openjdk.org/jdk/pull/11847