IEvent.Publish "forgets" that it's an IObservable when used with an anonymous record argument

[reinux]

When the argument for an event is an anonymous record, Event.Publish forgets that the IEvent it's returning is also an IObservable, requiring it to be cast to IObservable explicitly before it can be subscribed to or added to. Using Observable.add seems to work fine as well.

Of course, the workarounds are perfectly acceptable, but the type error is rather misleading and had me believing it simply can't be done.

Repro steps

let e = Event<_>()
e.Trigger {|number = 3|}
e.Publish.Add (printfn "%A")    // error

Expected behavior

e.Publish.Add should compile as the others do.

Actual behavior

error FS0193: Type constraint mismatch. The type 
    'IEvent<{|number : int|}>'    
is not compatible with type
    'IObservable<{|number : int|}>'    

Known workarounds

This works just fine when the IEvent is explicitly cast to IObservable, or when using a function rather than any of the IObservable instance methods.

(e.Publish :> IObservable<_>).Add(fun e -> printfn "Number: %d" e.number)   // works
e.Publish |> Observable.add(fun e -> printfn "Omfg: %d" e.number)   // works

Related information

F# 4.6, VS2019