Add support for @Async Kotlin function returning Unit?

Closes gh-31891

spring-context/src/test/kotlin/org/springframework/scheduling/annotation/AnnotationAsyncExecutionInterceptorKotlinTests.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright 2002-2023 the original author or authors.
+ *
+ * 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
+ *
+ *      https://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 org.springframework.scheduling.annotation
+
+import org.aopalliance.intercept.MethodInvocation
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.Test
+import org.mockito.BDDMockito.given
+import org.mockito.Mockito
+
+/**
+ * Kotlin tests for [AnnotationAsyncExecutionInterceptor].
+ *
+ * @author Sebastien Deleuze
+ */
+class AnnotationAsyncExecutionInterceptorKotlinTests {
+
+	@Test
+	fun nullableUnitReturnValue() {
+		val interceptor = AnnotationAsyncExecutionInterceptor(null)
+
+		class C { @Async fun nullableUnit(): Unit? = null }
+		val invocation = Mockito.mock<MethodInvocation>()
+		given(invocation.method).willReturn(C::class.java.getDeclaredMethod("nullableUnit"))
+
+		Assertions.assertThat(interceptor.invoke(invocation)).isNull()
+	}
+
+}