Wrong attempt to rebind a wildcard sequence compiles and results in runtime CCE

[som-snytt]

Minimized code

@main
def f = List(42) match { case List(xs @ (ys: _*)) => xs }

Output

$ ~/bin/dotc noseq.scala
-- [E029] Pattern Match Exhaustivity Warning: noseq.scala:3:12 -----------------
3 |def f = List(42) match { case List(xs @ (ys: _*)) => xs }
  |        ^^^^^^^^
  |        match may not be exhaustive.
  |
  |        It would fail on pattern case: List(_, _: _*), Nil

longer explanation available when compiling with `-explain`
one warning found
$ ~/bin/dotr f
Exception in thread "main" java.lang.ClassCastException: class scala.collection.immutable.$colon$colon cannot be cast to class java.lang.Integer (scala.collection.immutable.$colon$colon is in unnamed module of loader 'app'; java.lang.Integer is in module java.base of loader 'bootstrap')
        at scala.runtime.BoxesRunTime.unboxToInt(BoxesRunTime.java:99)
        at noseq$package$.f(noseq.scala:3)
        at f.main(noseq.scala:2)

Expectation

scala 2.13.1> def f = List(42) match { case List(xs @ (ys @ _*)) => xs }
                                                              ^
              error: bad simple pattern: bad use of _* (sequence pattern not allowed)

The exhaustivity warning is also misguided, also in the correct case.