Object spread operation typings are divorced from resultant runtime values.

[jasa-blad]

Bug Report

🔎 Search Terms

spread object

🕗 Version & Regression Information

v4.7.4 v4.8.0-beta

• This is a crash
• This changed between versions ______ and _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play?ts=4.7.4#code/MYewdgzgLgsAUAAhAIwFYFNhQIzwLwDe8CAFugDbkgBcARAOogBO5AJrfAL4ICGECoSFADc8MXEHRiKDFgBM+IojKUatALI8mEDnG58B4aKLjiAZgFcwWAJbgEAZQAOTdD1YAeAIIAaAEIAfAAUPNS+yNR+AJRKCK5QFkxgsQgAdOk8PsRp6chZiJxc4vCSsIgOFQ74zq7uQTKYOD4N8lEmAPTtCFAkNvwAnhSs-GDoAG7oTKnFcJ0I9CQ8UN1k3f1O6BD90OgAtis2YADWEPBzNW6evoEIeAEIXgBkfjNzC0txVlA2u+jtteQbFB+ghxjxyBYlptujQzl0Lu5vP57ncEAB5XZApEII7ofogMwIQLPIA

Playground link with relevant code

💻 Code

const
 object1
={
 hello:"World"
} as const;


const
 object2
={
 hello:"Mars"
} as const;


function Spread<A extends object, B extends object>(a:A,b:B){
 return{
  ...a,
  ...b,
 }
}


const
 SSSS
=Spread(object1,object2);
// this yelds never.


// What the typesystem thinks
// Spread<A,B> => A&B


// What runtime/reality evaluates to:
// Spread<A,B> => Omit<A, keyof B>&B

🙁 Actual behavior

The resultant type of the spread operation did not model that which is happening in runtime.

🙂 Expected behavior

The type system is grounded and models that which is happening in runtime.

[jasa-blad]

Its hard to implement infered types when the type system gets in the way, regardless of the robustness of being able to declare type dependencies in consuming functions or classes.

[RyanCavanaugh]

Duplicate #10727. Lacking a specific type operator to represent spread, ... uses intersection as the next-closest thing when one of the operands is a type parameter.