Microsoft/TypeScript

Only `null` and `undefined` included in later overloads during inference between overloads

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#47,586 opened on Jan 25, 2022

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 (2 comments) (2 reactions) (0 assignees)TypeScript (6,726 forks)batch import
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Description

Bug Report

🔎 Search Terms

overload null undefined

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I wrote the FAQ

⏯ Playground Link

Playground link with relevant code

💻 Code

This is a bit over-reduced but it's still weird

declare function callIt<T>(func: {
    (): T;
    (): T;
}): T;

declare function overloaded(): string;
declare function overloaded(): number | null;

// m: string | null. Where'd number go?
const m = callIt(overloaded);

🙁 Actual behavior

m: string | null

🙂 Expected behavior

m: string | number | null

This feels intentional since it only null and undefined make it out of the second signature, but I can't find where we do this or what the motivation behind it is.

Contributor guide

Research direction
Investigate the overload resolution algorithm in TypeScript's type checker to understand why only null and undefined are preserved in later overloads when there are multiple overloads with the same signature.
Tech stack
typescript
Domain
developer experience
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
TypeScriptGit
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25