Could not find proxy for implicit val (2.13.0)

[djspiewak]

• Error: https://gist.github.com/djspiewak/dc9890fccf13e2915ec9b20a2a85b40e
• Unminimized state: https://github.com/djspiewak/cats-effect/tree/18e28a6a018998586d73749e11468cf7dfa36186

Lines in question:

  checkAllAsync("IO", implicit ec => {
    implicit val cs = ec.contextShift[IO]
    ParallelTests[IO, IO.Par].parallel[Int, Int]
  })

This also fails:

  checkAllAsync("IO", implicit ec => {
    implicit val cs = ec.contextShift[IO]
    ParallelTests[IO].parallel[Int, Int]
  })

This compiles successfully:

  checkAllAsync("IO", implicit ec => {
    implicit val cs = ec.contextShift[IO]

    // do NOT inline this val; it causes the 2.13.0 compiler to crash for... reasons
    val module = ParallelTests[IO]
    module.parallel[Int, Int]
  })

We discovered this when trying to release Cats Effect 2.0. Feels minimizeable but it's hard to say right now. The definition of ParallelTests (the object) is the following:

object ParallelTests {
  type Aux[M[_], F0[_]] = ParallelTests[M] { type F[A] = F0[A]; val laws: ParallelLaws.Aux[M, F0] }

  def apply[M[_]](implicit ev: Parallel[M]): ParallelTests.Aux[M, ev.F] =
    apply[M, ev.F](ev, implicitly)

  def apply[M[_], F[_]](implicit ev: Parallel.Aux[M, F], D: DummyImplicit): ParallelTests.Aux[M, F] =
    new ParallelTests[M] { val laws = ParallelLaws[M] }
}

Sorry for not minimizing. Just wanted to report first and pare it down later.