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

[mheiber]

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:

• Typings for Function.prototype.bind were discussed in https://github.com/Microsoft/TypeScript/issues/212 but I couldn't find any mention of type predicates.

• Discussion on StackOverflow

[weswigham]

AFAIK, this should be fixable with some predicate-y overloads for the strictBindCallApply types in lib.d.ts.