Capture set parameters upper bounded by cap.rd cannot be widened to cap.rd

[natsukagami]

Compiler version

Latest main (23469a1)

Minimized code

import language.experimental.captureChecking
import caps.*

trait A

extension (a: A^{cap.rd})
  def await = ()

def awaitA[C <: {cap.rd}](a: A^{C}) = a.await

Output

-- [E007] Type Mismatch Error: ../dotty-test/upcast-rd.scala:11:38 -------------
11 |def awaitA[C <: {cap.rd}](a: A^{C}) = a.await
   |                                      ^
   |Found:    (a : test.upcastrd.A^{C})
   |Required: test.upcastrd.A^{cap.rd}
   |
   |where:    cap is a fresh root capability created in method awaitA when checking argument to parameter a of method await
   |
   | longer explanation available when compiling with `-explain`

Expectation

Should compile.

[natsukagami]

In the same vein, reach capabilities also don't work:

import language.experimental.captureChecking
import caps.*

trait Future[T]

extension [T](fut: Future[T]^{cap.rd})
  def await: T = ???

class Collector[T](val futs: Seq[Future[T]^{cap.rd}]):
  val results = Seq[Future[T]^{futs*}]()

extension [T](futs: Seq[Future[T]^{cap.rd}])
  def awaitAll =
    val collector = Collector(futs)
    collector.results.foreach((f: Future[T]^{futs*}) => f.await)
    futs.map(_.await)

gives

[error] ./fut.scala:17:57
[error] Found:    (f : test.fut.Future[T]^{futs*})
[error] Required: test.fut.Future[box T^?]^{cap.rd}
[error] 
[error] where:    cap is a fresh root capability created in anonymous function of type (f²: test.fut.Future[T]^{futs*}): T when checking argument to parameter fut of method await