Microsoft/TypeScript

type predicate-ness lost when using Function.prototype.bind

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#29,531 建立於 2019年1月22日

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Domain: lib.d.tsExperience EnhancementHelp WantedSuggestion

描述

TypeScript Version: 3.2.4 or @next as of 2019-01-22

Search Terms:

Code

const isArray: typeof Array.isArray = Array.isArray.bind(Array);

Expected behavior: Compiles.

Actual behavior:

Error message:

error TS2322: Type '(arg: any) => boolean' is not assignable to type '(arg: any) => arg is any[]'.
  Signature '(arg: any): boolean' must be a type predicate.

What seems to be happening is that the type predicate-ness of Array.isArray is lost when .bind is used. I tested with a few type predicate functions, and always get the same error.

Playground Link:

Not reproducible on playground, because it depends on a higher TS version.

In the playground, the type of the RHS is any, so there is no compilation error.

Related Issues:

貢獻者指南

技術棧
typescript
領域
backend
議題類型
bug
難度面向新貢獻者的預計實作難度，1 表示很小改動，5 表示專家級工作。
4
預計時間有經驗貢獻者完成調查、實作、測試並準備 pull request 的粗略時間範圍。
1-2 days
活動狀態議題目前的可參與程度：新鮮、活躍、陳舊、阻塞或等待維護者輸入。
stale
清晰度議題是否清楚說明預期改動、驗收標準和下一步。
clear
前置要求
TypeScript type system basicsUnderstanding of type predicates
新手友善度1-100 的估計分數，表示該議題對首次貢獻者的友善程度。
20
研究方向
This issue reports that Function.prototype.bind loses type predicate information. The fix likely requires modifying the TypeScript's lib.d.ts to add overloads for bind that preserve type predicates, or updating the type checker to handle bound functions with type predicates. Look into the overload declarations of bind in lib.es5.d.ts and the type checker logic for function types in checker.ts. The related issue #212 discusses bind typings but not type predicates. A solution might involve adding new overloads for bind that accept and return type predicate signatures.