Instantiating abstract types bounded by CapSet with Nothing crashes the capture checker

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Compiler version

Latest nightly.

Minimized code

import language.experimental.captureChecking
import caps._

trait Abstract:
    type C <: CapSet
    def boom(): Unit^{this.C^}

class Concrete extends Abstract:
    type C = Nothing
    def boom() = () // dotty.tools.dotc.cc.IllegalCaptureRef: Nothing

Output

The compiler crashes with dotty.tools.dotc.cc.IllegalCaptureRef: Nothing.