Using a generic type as a return type of an async method

Question

A previous question made me wonder why the following method would raise a compile time error:

The return type of an async method must be void, Task or

Answer 1

Three problems:

• Just because T is "Task or a derived type" doesn't mean that it's Task or Task. What would you expect if I called MyMethodAsync where MyCustomTask derives from Task?

• The compiler needs to know whether it's building a state machine returning Task or Task when it compiles the method - it uses different helper classes in the different cases

• If an async method has a return type of Task, any return statements can't specify a value; if it has a return type of Task any return statements must specify a value which is implicitly convertible to T. How can that work within MyMethodAsync? It's a bit like saying "my method is either void or returns a T - you can decide when you call it".

It's not clear what you're trying to achieve here, but basically this isn't going to work.