Broken type inference for union of function and object

[Danksa]

🔎 Search Terms

function union infer

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about Structural vs Instantiation-Based Inference

⏯ Playground Link

https://www.typescriptlang.org/play/?jsx=0#code/C4TwDgpgBAYgrgOwMbAJYHsEEMA2BhdAWzEwgWAB4AVAPigF4oAKMAJ3TAGcAuKKgSgZ1OwVqgQBzANwAoGUkwiowCEsbUaLdl14BvKADdccVbyoBtALoAaKJwg4IKCABMzUAL6D6dXTKgBUAoISrpGOCactvaOzi4eDFBsHJyygVCsEMBwrAhQAAYqIrwAJGHGqh7RDk4qblBlMbWuHvmyHrLBSgTEpOQAClicIgAW7HASI4kaTArEvPDIaJi4PSQIZJS0-AuIKBjY+ETrmxpCQcey8orAUGt9wACCE4Sb07Szx7tLB6vHDxodrA9stDvcNuQzgAyKD6VD1BBwQgAIwgrE85wA8siAFa1AB0Q04qAkCE+xFscPqAEYAAyefhXLq3LCJcGbQbDYBjdATEZMIrARnXEK3ZFs-4Qp4vTYC1RCqRAA

💻 Code

type FunctionalComponent<T> = (props: T) => string;

const test = <T>(props: { values: T[], selected: T }) => {
 const {values, selected} = props;
 return `test: ${values}, selected: ${selected}`;
};
const ComponentPassthrough = <T>(comp: FunctionalComponent<T>): FunctionalComponent<T> => comp;

const ComponentAugment = <T>(comp: FunctionalComponent<T>): FunctionalComponent<T> & { id: number } => Object.assign(comp, { id: 10 });

const a = ComponentPassthrough(test);
const b = ComponentAugment(test);

🙁 Actual behavior

Typescript cannot infer/passthrough the type parameter in ComponentAugment

🙂 Expected behavior

It should resolve ComponentAugment the same way it does with ComponentPassthrough, but with the added id property in this case. The expected type of b in this example should be:

(<T>(props: {
 values: T[];
 selected: T;
}) => string) & { id: number }

Additional information about the issue

No response

[jcalz]

Essentially a duplicate of #49505. TS has quite limited support for higher order function type inference; it's implemented at #30215 and mentions the very specific circumstances in which you can expect it to work. As soon as you do something like add that intersection, it stops working.

[typescript-bot]

This issue has been marked as "Duplicate" and has seen no recent activity. It has been automatically closed for house-keeping purposes.