app.message payload arg compatibility in TypeScript

The message argument in app.message listeners does not provide sufficient properties in TypeScript.

Property 'user' does not exist on type 'KnownEventFromType<"message">'. Property 'user' does not exist on type 'MessageChangedEvent'.ts(2339)

A workaround is to cast the message value by (message as GenericMessageEvent).user but needless to say, this is not great.

I ran into this today also. Trying to use the official example from https://slack.dev/bolt-js/concepts

// This will match any message that contains 👋
app.message(':wave:', async ({ message, say }) => {
  await say(`Hello, <@${message.user}>`);
});

and immediately getting a ts compile error. This is not a great first-time experience. Not even sure how to get it working.

Originally posted by @memark in https://github.com/slackapi/bolt-js/issues/826#issuecomment-830565064

Start by reproducing the official app.message example from the issue and inspect the TypeScript event types behind the message listener. Check why the message union omits user for this callback; done means the example compiles without a cast and the relevant message variants remain correctly typed.