Partial<> with generics is throwing: Type 'xxx' is not assignable to type 'Partial<T>'.

[jozefchutka]

🔎 Search Terms

partial, Type is not assignable to type 'Partial'.

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/C4TwDgpgBAKglgWwgEygXigbwFAEhiIQBcAdgK4IBGEATgNzYC+22AZmSQMYED2JUAcwjAACgEMaBMQBsAPDCgQAHsAglkAZ1iFkAPgAUASiLjJcGfN1Y8NYWRr9MBJEQAMzXAHpPscNADkmFDOxFCudFCM-lBwWiQ8wFBiGhpwAiRilNLQwDzBflD+plJyMLr+AHRMLOxcvPxCohIlAEzyiipqmtpIekYmzealVji4nHwaibYaZNLAA2YWZehYjAy407PAFSEr4TZ2DlCbc+veUADyANJMQA

💻 Code

type Timed = {
	time:number;
}

function getPartial<T extends Timed>():Partial<T> {
	return {time:0}
	// Type '{ time: 0; }' is not assignable to type 'Partial<T>'.
}

function getPartial2<T extends Timed>():Partial<T> {
	const result:Partial<T> = {};
	result.time = 0;
	return result;
	// OK
}

🙁 Actual behavior

compiler throws Type '{ time: 0; }' is not assignable to type 'Partial<T>' for getPartial() while it is happy to create object with the same data via getPartial2()

🙂 Expected behavior

I would expect:

• a more descriptive error
• getPartial() should succeed or at least same behavior for getPartial() and getPartial2()

Additional information about the issue

No response

[MartinJohns]

The error is correct for getPartial, because the compiler doesn't know what type T is, it may be that 0 is not a valid value for time.

The actual error is that you're allowed to assign 0 to result.time in getPartial2. There's definitely an issue for this, but I can't recall which one right now. But #9825 is related.

[jcalz]

For getPartial() there's the SO question Why can't I return a generic 'T' to satisfy a Partial?, and here there's #51645 and probably others

For the unsoundness, welcome to TypeScript 😅. When you index into a generic variable with a specific key, TS widens the generic to its constraint. That's usually safe for reading, but unsafe for writing. Not sure if there's a specific issue for this particular unsoundness.