+: extractor is not able to infer the return type for a SeqView

[Atry]

minimized code

val head1 +: _ = List(1).view
val _: Int = head1

When compiling with Dotty 0.21.0-RC1, it reports the following error:

Found:    (head1 : Any)
Required: Int

expectation

It should compile without error as Scala 2.13 did.

[Atry]

I guess it may be some problem in handling covariant types in extractors.

[odersky]

I believe dotc is correct here. This should not typecheck. If we analyze what should be the parameters of the +: unapply, we get

[Int, collection.View, collection.SeqView[Int]]

But these do not bounds-check:

-- [E057] Type Mismatch Error: i7879.scala:5:35 --------------------------------
5 |  scala.+:.unapply[Int, collection.View, collection.SeqView[Int]] // error
  |                                   ^
  |Type argument collection.View does not conform to upper bound [_] => scala.collection.Seq[?] 

If we look at the code produced by nsc, it looks problematic:

[[syntax trees at end of                     typer]] // i7879.scala
package <empty> {
  object Test extends scala.AnyRef {
    def <init>(): Test.type = {
      Test.super.<init>();
      ()
    };
    private[this] val head1: Int with A = (scala.collection.immutable.List.apply[Int](1).view: scala.collection.SeqView[Int] @unchecked) match {
      case scala.`package`.+:.unapply[Int with A, [_]scala.collection.View[_] with CC[_], scala.collection.SeqOps[A,CC,C] with scala.collection.SeqView[Int]](<unapply-selector>) <unapply> ((head1 @ _), _) => head1
    };
    <stable> <accessor> def head1: Int with A = Test.this.head1;
    <synthetic> <artifact> private[this] val x$1: Unit = (Test.this.head1: Int with A @unchecked) match {
      case (_: Int) => ()
    }
  }
}

Note the unbound parameters A, CC and C in the type arguments to unapply. So, I think this worked only by accident before and should be rejected.