Type constraint mismatch when reraising a caught exception in a task CE

[adam-c-anderson]

I have some code somewhat like the following: (actual types obfuscated)

// unit -> Task<MyClass option>
member this.DoThing ()=
    task {
        try
            let! result = something.doAsync ()
            return result // I know, this could've just been return!, please overlook
        with
        | ex ->
            logger.LogError(ex, "Error doing the thing")
            raise ex // I know this doesn't preserve the stack trace, please overlook
            return None // Won't compile without this line
    }

I am very confused by the compile-time behavior of this. Why must the with expression include a return after calling raise? If the return None is omitted, I get a compilation error like this:

  [FS0193] Type constraint mismatch. The type 
    'TaskCode<MyClass option,unit>'    
is not compatible with type
    'TaskCode<MyClass option,MyClass option>'

I found the docs that say TaskCode<,> is an abbreviation for resumable code, though that doesn't bring me closer to understanding. I understand why the try and with expressions must return the same type, but shouldn't that requirement be dismissed by (re)raising an exception? Would return None even get executed even though the compiler requires it?

[baronfel]

All you should have to do here is return the raise ex expression. raise (like throw, failwith and others), returns a generic type so that it can be used in situations like this, you just always have to return something out of a CE context.

[adam-c-anderson]

That certainly seems to have worked. Thanks!