I suppose you'd have to make

    SumAsync(F1(),f2());

Buy it's kind of intractable, isn't it? Your language has to assume order dependency or independency and specify the other. Most seem to stick with lexical ordering implies execution order.

I think some use curly brace scoping to break up dependency. I want to say kotlin does something like this.

This is why they say async is a viral pattern but IMO that's because you're adding specificity and function coloring is necessary and good.