Instance of Conversion is not found for a singleton type

[neko-kai]

minimized code

import scala.language.implicitConversions

trait Fixture[A] extends Conversion[0, A]

trait TestFramework[A] {
  def (testName: String) in (test: given (Fixture[A]) => Unit): Unit = ???
}

trait Greeter {
  def greet(name: String): String = s"Hello $name"
}

case class MyFixture(name: String, greeter: Greeter)

class MyTest extends TestFramework[MyFixture] {
  "say hello" in {
    assert(0.greeter.greet(0.name) == s"Hello ${0.name}")
  }
}

expectation

Expected Fixture Conversion to apply and attach names to 0, but got error:

[error] example.scala:48:14: value greeter is not a member of Int
[error]     assert(0.greeter.greet(0.name) == s"Hello ${0.name}")
[error]              ^

Using an implicit def conversion instead of Conversion works as expected:

implicit def fixture0[A](z: 0)(implicit f: Fixture[A]): A = f(z)

[smarter]

https://github.com/lampepfl/dotty/blob/0e880778908f0203636f9565f0009479ff1581fd/compiler/src/dotty/tools/dotc/typer/Implicits.scala#L172-L184 looks related, it's used on implicit conversion methods but not Conversion instances.