Bug Report

🔎 Search Terms

Argument of type is not assignable to parameter of type, object properties overlap

🕗 Version & Regression Information

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

The closest I could find in the FAQ is this https://github.com/Microsoft/TypeScript/wiki/FAQ#why-are-all-types-assignable-to-empty-interfaces

Type { color: "red", margin: 1 } has the required member of Props, so it may correctly be evaluated by TS as fitting, however, the error in the first place is then misleading.

⏯ Playground Link

same result with 4.8.0-beta

Playground link with relevant code

💻 Code

export const is = <TargetType,>() => <T extends TargetType>(arg: T): T => arg

type Props = { color: string }

let x = is<Props>()({ margin: 1 }) // ERROR: Argument of type '{ margin: number; }' is not assignable to parameter of type 'Props'. Object literal may only specify known properties, and 'margin' does not exist in type 'Props'.(2345)
let y = is<Props>()({ color: "red", margin: 1 }) // margin .... OK

Since {margin: 1} is clearly tolerated, the error given is misleading as the actual error is not in the presence of margin but in the absence of color, which is a property required by the Props type.

🙁 Actual behavior

Argument of type '{ margin: number; }' is not assignable to parameter of type 'Props'. Object literal may only specify known properties, and 'margin' does not exist in type 'Props'.(2345)

🙂 Expected behavior

Argument of type '{ margin: number; }' is not assignable to parameter of type 'Props'. Object literal does not specify properties required by 'Props'.(2345)