Well I can only speak to C# really but... The preferred pattern is to avoid long running tasks in the default thread pool as it's a shared resource. Long runni

Well I can only speak to C# really but... The preferred pattern is to avoid long running tasks in the default thread pool as it's a shared resource. Long running tasks should be executed somewhere else, either some new thread pool or just a new thread or just don't use async and make it your caller's problem. The point is that you don't want to consume the limited shared resource. In your case, what would happen? Would a new thread be created? Is it ok to use the default pool? Did the synchronous method writer even consider this? It's hard to say. Now, does this need to be enforced by the language? No. Developers could simply append "_Async" to make it clear a method can be put on the default thread pool or not. That's still a form a coloring though. Even if you consider it a choice by the caller, it's still a color that needs to be determined and you never want to exhaust your default thread pool. That said, there are other reasons why calling an async method should have different semantics than calling a normal sync method. Just saying that the "can use shared threads" aspect of async methods can probably live as a convention.