Improve error message when yield! was used but yield is needed and vice versa

For newcomers in elmish views it's sometimes not easy to understand when to use yield vs yield!. And the compiler is not really useful here:

let div _ _ = 1

let subView _ _ = [1; 2]



let view model dispatch =
   [
        yield! subView model dispatch
        yield! div [] []
   ]

@cartermp

@cartermp is this ameliorated by some of the changes in F#4.5 regarding yields etc. or not really?

Only if it involves subtyping, which I don't think is the case with Elmish code.

@cartermp

@cartermp @dsyme will this in any way be affected or ameliorated by the work going on to make yield optional?

I think so, though improper use of yield! will still give the same bad error message.

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