Before You File a Proposal Please Confirm You Have Done The Following...

• I have searched for related issues and found none that match my proposal.
• I have searched the current rule list and found no rules that match my proposal.
• I have read the FAQ and my problem is not listed.

My proposal is suitable for this project

• My proposal specifically checks TypeScript syntax, or it proposes a check that requires type information to be accurate.
• My proposal is not a "formatting rule"; meaning it does not just enforce how code is formatted (whitespace, brace placement, etc).
• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Description

We have a number of public APIs in our project that return things like string | Promise<string>. In fact, we have so many of these that we have a helper type called MaybePromise. I believe it would be better if these functions always returned a Promise. It would make a nicer API for callers because they would only have to worry about handling rejected promise errors and would not have to worry about errors that are thrown as well

I probably wouldn't want to apply this check to our whole project as internally we can have errors propagate in many functions, but I'd love to apply it to our public type definition file (.d.ts).

Fail Cases

export interface Emulator {
	platform(details: { config: any; prerender: PrerenderOption }): App.Platform | Promise<App.Platform>;
}

Pass Cases

export interface Emulator {
	platform(details: { config: any; prerender: PrerenderOption }): Promise<App.Platform>;
}

Additional Info

promise-function-async has a somewhat similar goal, but doesn't work for me because it ignores methods with explicit return types, which is one of the things I'd like to detect and disallow. It also has a ton of false positives on my existing codebase where it's almost always fine for me to just propagate errors upwards