more unsound pattern matcher behavior

[scabug]

import scala.language.reflectiveCalls

trait Mutable

trait A {
  type Foo <: { def bippy: Int }

  def f(x: Any) = x match {
    case x: Foo with Mutable => x.bippy
    case _                   => -1
  }
}

object Test extends A {
  trait Foo { def bippy = 5 }

  def main(args: Array[String]): Unit = println(f(new Mutable { }))
  // java.lang.NoSuchMethodException: Test$$anon$1.bippy()
}

There's a comment which led me to this discovery; I don't know what was being imagined when this was written, but an intersection of one checkable type and one uncheckable type does not add up to a checkable type. It adds up to an uncheckable type.

// if at least one of the types in an intersection is checkable, use the checkable ones
// this avoids problems as in run/matchonseq.scala, where the expected type is `Coll with scala.collection.SeqLike`
// Coll is an abstract type, but SeqLike of course is not

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-7721?orig=1
Reporter: @paulp

[scabug]

@paulp said:
Oh, and if it's not obvious, this compiles with zero warnings.

[SethTisue]

I'm calling this "fixed in Scala 3", since you get an unchecked warning:

-- Unchecked Warning: ----------------------------------------------------------
7 |    case x: Foo with Mutable => x.bippy
  |         ^
  |the type test for A.this.Foo & Mutable cannot be checked at runtime because it refers to an abstract type member or type parameter