Uninitialized field on creating implicit val for ContextShift using TestContext

[arbitrary-dev]

Reproduction

scala> :paste
// Entering paste mode (ctrl-D to finish)

import cats.effect._
import cats.effect.laws.util.TestContext

implicit val ec: TestContext = TestContext()
implicit val cs: ContextShift[IO] = ec.contextShift[IO]

// Exiting paste mode, now interpreting.

scala.UninitializedFieldError: Uninitialized field: <pastie>: 15
  at .cs(<pastie>:15)
  ... 44 elided

Not implicit

scala> :paste
// Entering paste mode (ctrl-D to finish)

import cats.effect._
import cats.effect.laws.util.TestContext

implicit val ec: TestContext = TestContext()
val cs: ContextShift[IO] = ec.contextShift[IO]

// Exiting paste mode, now interpreting.

import cats.effect._
import cats.effect.laws.util.TestContext
ec: cats.effect.laws.util.TestContext = cats.effect.laws.util.TestContext@77ec71c0
cs: cats.effect.ContextShift[cats.effect.IO] = cats.effect.laws.util.TestContext$$anon$4@566ea66

[LukaJCB]

This seems to me like a REPL bug, not related to cats-effect. If I run the same code in something like scastie it works fine:

https://scastie.scala-lang.org/yB4wNSoiTTi8VHJhME0xHQ

[arbitrary-dev]

Fixed in #764