Change the signature of Array.unapplySeq for the benefits of Scala 3

[smarter]

Fixes scala/scala3#14693, see discussion in the issue.

[julienrf]

IIUC, we change the signature of unapplySeq so that a typed pattern within a pattern sequence works. It seems to me that we are changing the type signature of unapplySeq to accommodate the internal way the Scala compiler handles such a combination of patterns.

What is the behavior of the following?

val xs = Array(1L, true)
val ys = Array(1L, 2L)

def matched(any: Any): Unit =
  any match
    case Array(x: Long, _*) => println(x)

matched(xs)
matched(ys)

I would expect both to match.

[smarter]

case Array(x: Long, *) => println(x)

This isn't valid syntax, I assume you mean:

case Array(x: Long, _*) => println(x)

What is the behavior of the following?

In Scala 3 before this PR, only the first matches. In Scala 3 after this PR, both match.

[smarter]

(and both still match in Scala 2 after this PR too)

[NthPortal]

I don't have any objection to this; at the same time, I'm not entirely convinced that the problem described by Julien lies here and not in the compiler

[smarter]

I'm not entirely convinced that the problem described by Julien lies here and not in the compiler

To restate what's going on: in general when typing the unapplySeq call we need to instantiate its type parameter as Any (especially in Julien's example, nothing else would fit). According to the specification, this means the pattern matching phase will generate a call to isInstanceOf[Array[Any]] which erases to instanceof Object[].
This PR changes the unapplySeq argument type so that we instead generate a call to isInstanceOf[Array[_ <: Any]] which erases to instanceof Object (because primitive arrays are subtypes of Array[_ <: Any] but not of Array[Any]).
I don't see what the compiler could be doing differently here, and I can't reproduce the Scala 2 compiler behavior because it relies on a compiler bug as mentioned in scala/scala3#14693 (comment).

[smarter]

in general when typing the unapplySeq call we need to instantiate its type parameter as Any

... is what I thought until now, but it turns out that in Scala 3 this behavior is specific to unapplies defined in Scala 2 classes, so this might in fact be a compiler bug, mea culpa :). Investigation in scala/scala3#14693 (comment)

[smarter]

Closing, superceded by scala/scala3#14766