Dotty-friendly changes for the laws module

laws/js/src/test/scala/cats/effect/internals/IOContextShiftTests.scala

@@ -16,12 +16,12 @@
 
 package cats.effect.internals
 
-import cats.effect.{BaseTestsSuite, IO}
+import cats.effect.{BaseTestsSuite, ContextShift, IO}
 import scala.util.Success
 
 class IOContextShiftTests extends BaseTestsSuite {
   testAsync("ContextShift[IO] instance based on implicit ExecutionContext") { implicit ec =>
-    val contextShift = ec.contextShift[IO]
+    val contextShift: ContextShift[IO] = ec.ioContextShift
 
     val f = contextShift.shift.map(_ => 1).unsafeToFuture()
     f.value shouldBe None

laws/jvm/src/test/scala/cats/effect/DeferredJVMTests.scala

@@ -33,14 +33,14 @@ class DeferredJVMParallelism4Tests extends BaseDeferredJVMTests(4)
 abstract class BaseDeferredJVMTests(parallelism: Int) extends AnyFunSuite with Matchers with BeforeAndAfter {
   var service: ExecutorService = _
 
-  implicit val context = new ExecutionContext {
+  implicit val context: ExecutionContext = new ExecutionContext {
     def execute(runnable: Runnable): Unit =
       service.execute(runnable)
     def reportFailure(cause: Throwable): Unit =
       cause.printStackTrace()
   }
 
-  implicit val cs = IO.contextShift(context)
+  implicit val cs: ContextShift[IO] = IO.contextShift(context)
   implicit val timer: Timer[IO] = IO.timer(context)
 
   before {

laws/jvm/src/test/scala/cats/effect/IOJVMTests.scala

@@ -94,7 +94,7 @@ class IOJVMTests extends AnyFunSuite with Matchers {
 
   test("parMap2 concurrently") {
     import scala.concurrent.ExecutionContext.Implicits.global
-    implicit val cs = IO.contextShift(global)
+    implicit val cs: ContextShift[IO] = IO.contextShift(global)
 
     val io1 = IO.shift *> IO(1)
     val io2 = IO.shift *> IO(2)
@@ -107,7 +107,7 @@ class IOJVMTests extends AnyFunSuite with Matchers {
 
   test("long synchronous loops that are forked are cancelable") {
     val thread = new AtomicReference[Thread](null)
-    implicit val ec = new ExecutionContext {
+    implicit val ec: ExecutionContext = new ExecutionContext {
       def execute(runnable: Runnable): Unit = {
         val th = new Thread(runnable)
         if (!thread.compareAndSet(null, th))
@@ -122,7 +122,7 @@ class IOJVMTests extends AnyFunSuite with Matchers {
       val latch = new java.util.concurrent.CountDownLatch(1)
       def loop(): IO[Int] = IO.suspend(loop())
 
-      implicit val ctx = IO.contextShift(ec)
+      implicit val ctx: ContextShift[IO] = IO.contextShift(ec)
       val task = IO.shift *> IO(latch.countDown()) *> loop()
       val c = task.unsafeRunCancelable {
         case Left(e) => e.printStackTrace()

laws/jvm/src/test/scala/cats/effect/MVarJVMTests.scala

@@ -101,14 +101,14 @@ class MVarFullJVMParallelism4Tests extends BaseMVarJVMTests(4) {
 abstract class BaseMVarJVMTests(parallelism: Int) extends AnyFunSuite with Matchers with BeforeAndAfter {
   var service: ExecutorService = _
 
-  implicit val context = new ExecutionContext {
+  implicit val context: ExecutionContext = new ExecutionContext {
     def execute(runnable: Runnable): Unit =
       service.execute(runnable)
     def reportFailure(cause: Throwable): Unit =
       cause.printStackTrace()
   }
 
-  implicit val cs = IO.contextShift(context)
+  implicit val cs: ContextShift[IO] = IO.contextShift(context)
   implicit val timer: Timer[IO] = IO.timer(context)
 
   before {

laws/jvm/src/test/scala/cats/effect/SemaphoreJVMTests.scala

@@ -35,14 +35,14 @@ class SemaphoreJVMParallelism4Tests extends BaseSemaphoreJVMTests(4)
 abstract class BaseSemaphoreJVMTests(parallelism: Int) extends AnyFunSuite with Matchers with BeforeAndAfter {
   var service: ExecutorService = _
 
-  implicit val context = new ExecutionContext {
+  implicit val context: ExecutionContext = new ExecutionContext {
     def execute(runnable: Runnable): Unit =
       service.execute(runnable)
     def reportFailure(cause: Throwable): Unit =
       cause.printStackTrace()
   }
 
-  implicit val cs = IO.contextShift(context)
+  implicit val cs: ContextShift[IO] = IO.contextShift(context)
   implicit val timer: Timer[IO] = IO.timer(context)
 
   before {

laws/shared/src/main/scala/cats/effect/laws/util/TestContext.scala

@@ -135,6 +135,11 @@ final class TestContext private () extends ExecutionContext { self =>
     lastReportedFailure = None
   )
 
+  /**
+   * Derives a `cats.effect.Timer` from this `TestContext` for `IO`.
+   */
+  def ioTimer: Timer[IO] = timer[IO](IO.ioEffect)
+
   /**
    * Derives a `cats.effect.Timer` from this `TestContext`, for any data
    * type that has a `LiftIO` instance.
@@ -165,9 +170,14 @@ final class TestContext private () extends ExecutionContext { self =>
       }
     }
 
+  /**
+   * Derives a `cats.effect.ContextShift` from this `TestContext` for `IO`.
+   */
+  def ioContextShift: ContextShift[IO] = contextShift[IO](IO.ioEffect)
+
   /**
    * Derives a `cats.effect.ContextShift` from this `TestContext`, for any data
-   * type that has a `LiftIO` and `MonadError` instance.
+   * type that has an `Async` instance.
    *
    * Example:
    *

laws/shared/src/test/scala/cats/effect/ConcurrentCancelableFTests.scala

@@ -27,7 +27,7 @@ import scala.util.Success
 
 class ConcurrentCancelableFTests extends BaseTestsSuite {
   testAsync("Concurrent.cancelableF works for immediate values") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     check { (value: Either[Throwable, Int]) =>
       val received = Concurrent.cancelableF[IO, Int](cb => IO { cb(value); IO.unit })
@@ -36,7 +36,7 @@ class ConcurrentCancelableFTests extends BaseTestsSuite {
   }
 
   testAsync("Concurrent.cancelableF works for async values") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     check { (value: Either[Throwable, Int]) =>
       val received = Concurrent.cancelableF[IO, Int] { cb =>
@@ -48,7 +48,7 @@ class ConcurrentCancelableFTests extends BaseTestsSuite {
 
   testAsync("Concurrent.cancelableF can delay callback") { implicit ec =>
     import scala.concurrent.duration._
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val timer = ec.timer[IO]
 
     val task = Concurrent.cancelableF[IO, Int] { cb =>
@@ -70,7 +70,7 @@ class ConcurrentCancelableFTests extends BaseTestsSuite {
 
   testAsync("Concurrent.cancelableF can delay task execution") { implicit ec =>
     import scala.concurrent.duration._
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val timer = ec.timer[IO]
 
     val complete = Promise[Unit]()
@@ -89,7 +89,7 @@ class ConcurrentCancelableFTests extends BaseTestsSuite {
   }
 
   testAsync("Concurrent.cancelableF can yield cancelable tasks") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     val task = for {
       d <- MVar.empty[IO, Unit]
@@ -114,8 +114,8 @@ class ConcurrentCancelableFTests extends BaseTestsSuite {
   testAsync("Concurrent.cancelableF executes generated task uninterruptedly") { implicit ec =>
     import scala.concurrent.duration._
 
-    implicit val cs = ec.contextShift[IO]
-    implicit val timer = ec.timer[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
+    implicit val timer: Timer[IO] = ec.timer[IO]
 
     var effect = 0
     val task = Concurrent.cancelableF[IO, Unit] { cb =>

laws/shared/src/test/scala/cats/effect/ConcurrentContinualTests.scala

@@ -24,8 +24,8 @@ import scala.util.Success
 class ConcurrentContinualTests extends BaseTestsSuite {
   testAsync("Concurrent.continual allows interruption of its input") { implicit ec =>
     import scala.concurrent.duration._
-    implicit val cs = ec.contextShift[IO]
-    implicit val timer = ec.timer[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
+    implicit val timer: Timer[IO] = ec.timer[IO]
 
     val task = Ref[IO].of(false).flatMap { canceled =>
       (IO.never: IO[Unit])
@@ -42,8 +42,8 @@ class ConcurrentContinualTests extends BaseTestsSuite {
 
   testAsync("Concurrent.continual backpressures on finalizers") { implicit ec =>
     import scala.concurrent.duration._
-    implicit val cs = ec.contextShift[IO]
-    implicit val timer = ec.timer[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
+    implicit val timer: Timer[IO] = ec.timer[IO]
 
     val task = Ref[IO].of(false).flatMap { canceled =>
       (IO.never: IO[Unit])
@@ -63,7 +63,7 @@ class ConcurrentContinualTests extends BaseTestsSuite {
   }
 
   testAsync("Concurrent.continual behaves like flatMap on the happy path") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val task = IO(1).continual(n => IO(n.toOption.get + 1)).map(_ + 1)
 
     val f = task.unsafeToFuture()
@@ -72,7 +72,7 @@ class ConcurrentContinualTests extends BaseTestsSuite {
   }
 
   testAsync("Concurrent.continual behaves like handleErrorWith on errors") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     val ex = new Exception("boom")
     val task = IO.raiseError[Unit](ex).continual(n => IO(n.swap.toOption.get))
@@ -83,7 +83,7 @@ class ConcurrentContinualTests extends BaseTestsSuite {
   }
 
   testAsync("Concurrent.continual propagates errors in the continuation") { implicit ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     val ex = new Exception("boom")
     val task = IO(1).continual(_ => IO.raiseError(ex)).handleError(identity)
@@ -95,8 +95,8 @@ class ConcurrentContinualTests extends BaseTestsSuite {
 
   testAsync("Concurrent.continual respects the continual guarantee") { implicit ec =>
     import scala.concurrent.duration._
-    implicit val cs = ec.contextShift[IO]
-    implicit val timer = ec.timer[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
+    implicit val timer: Timer[IO] = ec.timer[IO]
 
     val task = Ref[IO].of(false).flatMap { completed =>
       IO.unit

laws/shared/src/test/scala/cats/effect/ContextShiftTests.scala

@@ -30,7 +30,7 @@ class ContextShiftTests extends BaseTestsSuite {
   type IorTIO[A] = IorT[IO, Int, A]
 
   testAsync("ContextShift[IO].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
 
     val f = cs.shift.unsafeToFuture()
     f.value shouldBe None
@@ -40,7 +40,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("ContextShift[IO].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val ec2 = TestContext()
 
     val f = cs.evalOn(ec2)(IO(1)).unsafeToFuture()
@@ -56,7 +56,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("ContextShift.evalOnK[IO]") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val ec2 = TestContext()
 
     val funK = ContextShift.evalOnK[IO](ec2)
@@ -75,7 +75,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- EitherT
 
   testAsync("Timer[EitherT].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[EitherTIO]].shift.value.unsafeToFuture()
 
     f.value shouldBe None
@@ -85,7 +85,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[EitherT].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[EitherTIO]]
     val ec2 = TestContext()
 
@@ -104,7 +104,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- OptionT
 
   testAsync("Timer[OptionT].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[OptionTIO]].shift.value.unsafeToFuture()
 
     f.value shouldBe None
@@ -114,7 +114,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[OptionT].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[OptionTIO]]
     val ec2 = TestContext()
 
@@ -133,7 +133,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- WriterT
 
   testAsync("Timer[WriterT].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[WriterTIO]].shift.value.unsafeToFuture()
 
     f.value shouldBe None
@@ -143,7 +143,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[WriterT].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[WriterTIO]]
     val ec2 = TestContext()
 
@@ -162,7 +162,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- StateT
 
   testAsync("Timer[StateT].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[StateTIO]].shift.run(0).unsafeToFuture()
 
     f.value shouldBe None
@@ -172,7 +172,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[StateT].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[StateTIO]]
     val ec2 = TestContext()
 
@@ -191,7 +191,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- Kleisli
 
   testAsync("Timer[Kleisli].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[KleisliIO]].shift.run(0).unsafeToFuture()
 
     f.value shouldBe None
@@ -201,7 +201,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[Kleisli].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[KleisliIO]]
     val ec2 = TestContext()
 
@@ -220,7 +220,7 @@ class ContextShiftTests extends BaseTestsSuite {
   // -- IorT
 
   testAsync("Timer[IorT].shift") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val f = implicitly[ContextShift[IorTIO]].shift.value.unsafeToFuture()
 
     f.value shouldBe None
@@ -230,7 +230,7 @@ class ContextShiftTests extends BaseTestsSuite {
   }
 
   testAsync("Timer[IorT].evalOn") { ec =>
-    implicit val cs = ec.contextShift[IO]
+    implicit val cs: ContextShift[IO] = ec.ioContextShift
     val cs2 = implicitly[ContextShift[IorTIO]]
     val ec2 = TestContext()