Returing a generic argument from a function allows returning a wrong type

[StyleShit]

🔎 Search Terms

• Wrong generic return type
• Function generic return type mismatch
• Returning a generic allows returning a wrong type

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about returning a generic type

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.5.3#code/GYVwdgxgLglg9mABKSAeAKogpgDylsAEwGdEBvRABwCc5KAuRARkQF8A+ACgENqBzRugCUg8gChEialighqSMhMmIAdGt58ANEsk06jAORg4UALRMD2yawDcY1mLEQExKIgBu3ADYgsiALzI4BCcFHoMzGxCdsqIAPRxiAB6APxiQA

💻 Code

function func<T extends { prop: 1 }>(arg: T): T {
  return {
    ...arg,
    prop: 'not-1',
  };
}

const value = func({ prop: 1 });
    // ^? { prop: 1 }

🙁 Actual behavior

The function allows returning a wrong type, which results in an error in runtime

🙂 Expected behavior

A compile error in the return statement

Additional information about the issue

It actually started while me & @IdoBouskila were researching a similar bug (playground):

type Fruit =
  | {
    name: string;
    count: 1 | 2 | 3;
    available: true;
  }
  | {
    name: string;
    count: 0;
  }

function func<T extends Fruit>(fruit: T): T {
  return {
    ...fruit,
    count: fruit.count + 1,
  };
}

const value = func({ name: 'apple', count: 0 });
    // ^? { name: 'apple', count: 0 }

In addition, when removing the return type, the inferred return type is still unexpected (playground):

function func<T extends { prop: 1 }>(arg: T) {
  return {
    ...arg,
    prop: 2,
  };
}

const value = func({ prop: 1 });

const prop = value.prop;
    // ^? 1

I would've expected it to be number.

Am I missing something here?

[MartinJohns]

Duplicate of #50559 / #50185 / #10727. Used search terms: generic spread return in:title

[typescript-bot]

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