Enhancement: [promise-function-async] flag non-async functions returning unions with non-Promise types

[ericbf]

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

• I believe my proposal would be useful to the broader TypeScript community (meaning it is not a niche proposal).

Link to the rule's documentation

https://typescript-eslint.io/rules/promise-function-async/

Description

I am finding cases in my code where it accidentally is returning undefined or some other guard type (null, etc) from a function that we expected to be async, but the rule promise-function-async is not flagging those as I expected it to (allowing us to catch those mistakes).

Please add an option to promise-function-async that will allow it to flag any function that returns promises at all, either as the main return type, or as a union containing promises.

Fail

function getSomething(id?: string) {
    if (!id) {
        return undefined
    }

    return db.getByID(id)
}

Pass

async function getSomething(id?: string) {
    if (!id) {
        return undefined
    }

    return db.getByID(id)
}

Additional Info

There is an issue for the opposite of my request, #1005. I’d like that feature back (probably behind an option, seeing as it was designated as a bug before).

[bradzacher]

I wonder if there's a heuristic we can apply here to avoid the need for an option.

Like "if the function has an explicit or contextual return type that is not Promise<T>, then ignore it, else if the implicit return type includes Promise at all, report it"

[ericbf]

If it has an explicit type that includes non-promises, that would make sense, sure. The real issue, I suppose, is when the implicit type includes a union with non-promises, it should still be flagged, imo.

[ericbf]

@bradzacher I put up a PR implementing the heuristic you suggested. Take a look and let me know what you think, or of any suggestions.