Compiler version

3bea2f3

Minimized code

import scala.language.experimental.captureChecking
import scala.language.experimental.separationChecking

trait Foo extends caps.ExclusiveCapability
trait Bar extends Foo, caps.Mutable
object Foo:
  extension (consume self: Foo)
    def sink: Unit = ()

def Test =
  val x: Bar^ = new Bar {}
  x.sink
  x.sink // should error?

Output

Compiles

Expectation

I expect the second call to x.sink to fail separation checking because x is a mutable reference (Bar^).
If we change x to val x: Foo = new Bar {}, then separation checking works as expected and the second call to x.sink fails.
I don't expect separation checking to fail if we had val x: Bar = new Bar {} (which works as expected).