Microsoft/TypeScript

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

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#29,531 opened on Jan 22, 2019

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Domain: lib.d.tsExperience EnhancementHelp WantedSuggestion

Description

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:

Contributor guide

Tech stack
typescript
Domain
backend
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.
1-2 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
TypeScript type system basicsUnderstanding of type predicates
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
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.