Merge pull request #2228 from armanbilge/thunk-ce2

Backport thunk

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

@@ -1152,7 +1152,7 @@ object IO extends IOInstances {
    * `IO`.
    */
   def defer[A](thunk: => IO[A]): IO[A] = {
-    val nextIo = Suspend(() => thunk)
+    val nextIo = Suspend(Thunk.asFunction0(thunk))
     if (isFullStackTracing) {
       IOTracing.decorated(nextIo)
     } else {
@@ -1167,7 +1167,7 @@ object IO extends IOInstances {
    * into the `IO`.
    */
   def delay[A](body: => A): IO[A] = {
-    val nextIo = Delay(() => body)
+    val nextIo = Delay(Thunk.asFunction0(body))
     if (isFullStackTracing) {
       IOTracing.decorated(nextIo)
     } else {

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

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2017-2021 The Typelevel Cats-effect Project Developers
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+/**
+ * A utility to convert a by-name `thunk: => A` to a `Function0[A]` (its binary representation).
+ * Scala 2 performs this optimization automatically but on Scala 3 the thunk is wrapped inside of a new `Function0`.
+ * See https://github.com/typelevel/cats-effect/pull/2226
+ */
+private object Thunk {
+  private[this] val impl =
+    ((x: Any) => x).asInstanceOf[(=> Any) => Function0[Any]]
+
+  def asFunction0[A](thunk: => A): Function0[A] = impl(thunk).asInstanceOf[Function0[A]]
+}

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

@@ -0,0 +1,27 @@
+/*
+ * Copyright 2020-2021 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.effect
+
+class ThunkSpec extends CatsEffectSuite {
+
+  test("Thunk.asFunction0") {
+    var i = 0
+    val f = () => i += 1
+    assertEquals(Thunk.asFunction0(f()), f)
+  }
+
+}