Ideally, I would just check messageSpecs[message.type].canBeSidebarSource. Doing this though causes several flow issues throughout the message-fetchers.js since type refinement doesn't occur without explicitly checking for types.

I couldn't think of a better solution to the Flow issue, but we're safeguarded by the unit tests testing both isInvalidSidebarSource and messageSpec.canBeSidebarSource anyways

Update: I think this check can actually be simplified back down to !messageSpecs[message.type].canBeSidebarSource once ENG-5215 is done since that should remove the use case of this function in message-fetchers. For now to pass flow checks I'll need to keep it though (ENG-5215 will come after this in the stack, so I'll fix this check then if it can be done)

Thanks for the suggestion, though when I tried it I got the same Flow warnings as just doing

function isInvalidSidebarSource(
  message: RawMessageInfo | MessageInfo,
): boolean {
    return messageSpecs[message.type].canBeSidebarSource;
}