Remove Thunk.asFunction0 utility

build.sbt

@@ -648,7 +648,10 @@ lazy val core = crossProject(JSPlatform, JVMPlatform, NativePlatform)
       ProblemFilters.exclude[IncompatibleResultTypeProblem](
         "cats.effect.unsafe.TimerSkipList.insert"),
       ProblemFilters.exclude[IncompatibleResultTypeProblem](
-        "cats.effect.unsafe.WorkerThread.sleep")
+        "cats.effect.unsafe.WorkerThread.sleep"),
+      // #3787, internal utility that was no longer needed
+      ProblemFilters.exclude[MissingClassProblem]("cats.effect.Thunk"),
+      ProblemFilters.exclude[MissingClassProblem]("cats.effect.Thunk$")
     ) ++ {
       if (tlIsScala3.value) {
         // Scala 3 specific exclusions

core/jvm/src/main/scala/cats/effect/IOCompanionPlatform.scala

@@ -47,14 +47,14 @@ private[effect] abstract class IOCompanionPlatform { this: IO.type =>
    * Implements [[cats.effect.kernel.Sync.blocking]].
    */
   def blocking[A](thunk: => A): IO[A] = {
-    val fn = Thunk.asFunction0(thunk)
+    val fn = () => thunk
     Blocking(TypeBlocking, fn, Tracing.calculateTracingEvent(fn.getClass))
   }
 
   // this cannot be marked private[effect] because of static forwarders in Java
   @deprecated("use interruptible / interruptibleMany instead", "3.3.0")
   def interruptible[A](many: Boolean, thunk: => A): IO[A] = {
-    val fn = Thunk.asFunction0(thunk)
+    val fn = () => thunk
     Blocking(
       if (many) TypeInterruptibleMany else TypeInterruptibleOnce,
       fn,
@@ -80,7 +80,7 @@ private[effect] abstract class IOCompanionPlatform { this: IO.type =>
    * Implements [[cats.effect.kernel.Sync.interruptible[A](thunk:=>A):*]]
    */
   def interruptible[A](thunk: => A): IO[A] = {
-    val fn = Thunk.asFunction0(thunk)
+    val fn = () => thunk
     Blocking(TypeInterruptibleOnce, fn, Tracing.calculateTracingEvent(fn.getClass))
   }
 
@@ -104,7 +104,7 @@ private[effect] abstract class IOCompanionPlatform { this: IO.type =>
    * Implements [[cats.effect.kernel.Sync!.interruptibleMany]]
    */
   def interruptibleMany[A](thunk: => A): IO[A] = {
-    val fn = Thunk.asFunction0(thunk)
+    val fn = () => thunk
     Blocking(TypeInterruptibleMany, fn, Tracing.calculateTracingEvent(fn.getClass))
   }
 

core/shared/src/main/scala/cats/effect/IO.scala

@@ -1130,7 +1130,7 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
    * Any exceptions thrown by the effect will be caught and sequenced into the `IO`.
    */
   def delay[A](thunk: => A): IO[A] = {
-    val fn = Thunk.asFunction0(thunk)
+    val fn = () => thunk
     Delay(fn, Tracing.calculateTracingEvent(fn))
   }
 

tests/shared/src/test/scala/cats/effect/ThunkSpec.scala

@@ -18,11 +18,11 @@ package cats.effect
 
 class ThunkSpec extends BaseSpec {
 
-  "Thunk.asFunction0" should {
+  "IO.delay" should {
     "return the same function" in {
       var i = 0
       val f = () => i += 1
-      Thunk.asFunction0(f()) eq f
+      IO.delay(f()).asInstanceOf[IO.Delay[Unit]].thunk eq f
     }
   }