Re: RpcPromise, I'm pretty sure all logical operations will result in unexpected results. TypeScript isn't going to complain about using RpcPromise as a boolean.

Maybe the best solution is just an eslint plugin. Like this plugin basically warns for the same thing on another type: https://github.com/bensaufley/eslint-plugin-preact-signals

Overloading .map() does feel a bit too clever here, as it has this major difference from Array.map. I'd rather see it as .mapRemote() or something that immediately sticks out.

I can imagine a RpcPromise.filterRemote(func: (p: RPCPromise) => RPCPromise) that only allows filtering on the truthiness of properties; in that case the types really would save someone from confusion.

I guess if the output type of map was something like:

    type MapOutput = RpcPromise | MapOutput[] | Record<string, MapOutput>;
    map(func: (p: RpcPromise) => MapOutput)

Though another case occurs to me that might not be caught by any of this:

    result = aPromise.map(friend => {...friend, nickname: getNickname(friend.id, userId)})