the type you're seeing isn't an

Output<string>

it's a

function that returns Output<string>

. you need to either call the function or change the class method to a

get

. I.e. for the latter:

export class CustomResource extends ComponentResource {
    public get resourceArn(): Output<string> {
        return this.resource.arn;
    }
}

omg thanks for spotting this so easily. can't believe I missed it