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Calling an overloaded function should resolve to best match, not first #48077
Description
Bug Report
🔎 Search Terms
function overload signature void promise
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about overloads
⏯ Playground Link
Playground link with relevant code
💻 Code
declare const it: {
(callback: () => void): void;
(callback: () => Promise<void>): void;
};
it(async () => { });🙁 Actual behavior
Autocomplete and hover show the first matching signature:
(callback: () => void): void;🙂 Expected behavior
I would have hoped for the more precise signature:
(callback: () => Promise<void>): void;More (Downstream) Details
This actually causes some issues in typescript-eslint's no-misused-promises rule. See typescript-eslint/typescript-eslint#4620:
- When someone calls
it(like above, we'd want to be able to check the precise resolved signature for the call to know whether they're dangerously passing a() => Promise<void>to an argument that takes() => void - The type checker
getResolvedSignatureAPI returns the() => voidoverload instead first - That overload would make calls that provide
() => Promise<void>seem unsafe
Our workaround is to check all call signatures, which is imprecise and can lead to false negatives in cases like:
declare const it: {
(syncName: string, callback: () => void): void;
(asyncName: number, callback: () => Promise<void>): void;
};
// We'll see that an overload exists that has callback: () => Promise<void>.
// Without knowing it's the specific resolved one for this call,
// we'll consider the call safe (even though it's not)
it('', async () => { });See standalone repro here: https://github.com/JoshuaKGoldberg/repros/tree/typescript-promise-returning-overload-resolve
I think this is different from #41563 because this issue also repros in 4.1.0-beta.
Activity
JoshuaKGoldberg commentedon Mar 1, 2022
tjjfvi commentedon Mar 1, 2022
MartinJohns commentedon Mar 2, 2022
This is not a bug, it's working as intended. You're supposed to write the order of signatures from most specific to least specific. https://www.typescriptlang.org/docs/handbook/declaration-files/do-s-and-don-ts.html#ordering
You might want to rephrase it as a feature request instead of a bug report.
JoshuaKGoldberg commentedon Mar 2, 2022
Aha, thank you - I had forgotten/missed that. I guess changing it would be a pretty breaking change - will close this issue and instead work on fixing the Mocha type definitions. Cheers!
RyanCavanaugh commentedon Mar 2, 2022
We tried various "best" algorithms at first, and they were just disasters, even before generics were in the mix. Ask the C# team about overload resolution if you want to hear a rant :)