Microsoft/TypeScript

Adding an overload to .filter breaks specific inference with nested functions

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#56,013 opened on Oct 6, 2023

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BugDomain: check: Type InferenceHelp Wanted

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Description

🔎 Search Terms

"filter inference overload", "filter boolean"

🕗 Version & Regression Information

  • This is the behavior in every version I tried, and I reviewed the FAQ for entries about inferred types

⏯ Playground Link

https://tsplay.dev/wQbrvN

💻 Code

type NonFalsy<T> = T extends false | 0 | "" | null | undefined | 0n
    ? never
    : T;

// Comment this out
interface Array<T> { filter(predicate: BooleanConstructor, thisArg?: any): NonFalsy<T>[]; }

const id = <T,>() => (t: T) => !!t;

['foo', 'bar'].filter(id())
//                    ^?

🙁 Actual behavior

Nested function inference works (type parameter is string) without the additional overload, and fails (type parameter is unknown) with the overload (even though the added overload isn't the one in use).

🙂 Expected behavior

Nested function inference should work regardless of whether an overload is added to .filter or not.

Additional information about the issue

Adding the overload is described in https://github.com/microsoft/TypeScript/issues/50387 and made popular by ts-reset. It's possible the overload order may matter in this instance, but difficult to test when the main overloads are coming from lib definitions.

Contributor guide

Research direction
Investigate how the overloaded filter signature affects type inference for nested functions; check if the new overload is being incorrectly selected or interfering with inference; examine the overload resolution order and see if reordering helps.
Tech stack
typescript
Domain
compilers
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitTypeScriptcompiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30