Enforce Future/void return declaration for async methods

Closes gh-27734

spring-aop/src/main/java/org/springframework/aop/interceptor/AsyncExecutionAspectSupport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -286,10 +286,14 @@ else if (ListenableFuture.class.isAssignableFrom(returnType)) {
 		else if (Future.class.isAssignableFrom(returnType)) {
 			return executor.submit(task);
 		}
-		else {
+		else if (void.class == returnType) {
 			executor.submit(task);
 			return null;
 		}
+		else {
+			throw new IllegalArgumentException(
+					"Invalid return type for async method (only Future and void supported): " + returnType);
+		}
 	}
 
 	/**