Multicasting values into a subject can change the type of the observable sequence

[glopesdev]

To reproduce, consider the below workflow where Subject is of type object. Surprisingly the type of the Int source will change to object following the MulticastSubject node, which fundamentally violates the contract of Sink operators.

Copy workflow

<?xml version="1.0" encoding="utf-8"?>
<WorkflowBuilder Version="2.8.4"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xmlns:rx="clr-namespace:Bonsai.Reactive;assembly=Bonsai.Core"
                 xmlns:sys="clr-namespace:System;assembly=mscorlib"
                 xmlns="https://bonsai-rx.org/2018/workflow">
  <Workflow>
    <Nodes>
      <Expression xsi:type="Combinator">
        <Combinator xsi:type="IntProperty">
          <Value>0</Value>
        </Combinator>
      </Expression>
      <Expression xsi:type="MulticastSubject">
        <Name>Output</Name>
      </Expression>
      <Expression xsi:type="rx:BehaviorSubject" TypeArguments="sys:Object">
        <rx:Name>Output</rx:Name>
      </Expression>
    </Nodes>
    <Edges>
      <Edge From="0" To="1" Label="Source1" />
    </Edges>
  </Workflow>
</WorkflowBuilder>

[glopesdev]

Interestingly, this issue can be traced all the way back to the Do operator from System.Reactive:
dotnet/reactive#2147