Narrowing with a typeguard fails after object spread

[Akiyamka]

🔎 Search Terms

typeguard narrowing, object spread

🕗 Version & Regression Information

This is the behavior in every version I tried, and I recheck handbook about:

• https://www.typescriptlang.org/docs/handbook/2/narrowing.html#using-type-predicates
• https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.2.2#code/JYOwLgpgTgZghgYwgAgCIBUDyyDeAoZZGAe2IC5kBnMKUAcwG4DkAjOKAfgutpEbwC+ePKEixEKDJgDqwMAAsAQu2QQAHpBAATSmiy5mbKNxr0mQvAmIhqqmwFcoEZVACSlAMoQwyALzIACi0wcj1MAEo-AD5kAEJY4OIAOiMmPAB6dOQPAAcnOC1kBHzIXThWezo8GHsQBDBga2QAWzhQALUKKXCurFkFFwNCYBhA2LUU9kj8QkIJoz9kAHItCHh7ABswJaZCC0IIBycXdy8wDvC0wisbHwBPRfn2BmRM5AfgXR56WOZmJzAjhAQ1mSTBamYQgsb368mI9h8lDyEAKyAA7sQoABrMq6dQ5CD1CBaaq1eqNYEsBEdXoRWmwwYzZAjMZPKDTZhzSZQRYrNZwTbbXbIfZ2SiOZzsU7eC5XIrWWwPfxsl6vLIfL6mPi-Qj-bxAkGEEihCbGgA0nNY7AobMhgmEQA

💻 Code

interface DTO {
  foo: string;
  bar?: string;
}

interface DTOWithBar extends DTO {
  bar: string;
}

const ensureBarIsSet = (dto: DTO) => !!dto.bar;

function main(x: DTO): DTOWithBar {
  if (!x.bar) {
    x.bar = 'default';
  }

  ensureBarIsSet(x);
  const y = x.bar; // y is string!
  return {   ...x  } 
  //         ^^^^
  // Type 'undefined' is not assignable to type 'string'
}

🙁 Actual behavior

Typescript ignore type guard if spread used

🙂 Expected behavior

Typescript respect type guard even after spread

Additional information about the issue

No response

[fatcerberus]

More accurately, duplicate of #42384 (there’s no discriminated union here).

[Akiyamka]

From #42384:

Type guards do not propagate type narrowings to parent objects. The narrowing is only applied upon access of the narrowed property which is why the destructing function works, but the reference function does not. Narrowing the parent would involve synthesizing new types which would be expensive.

Not sure it's my case, I already declare new type (DTOWithBar), and just want to keep it

[fatcerberus]

It’s the same thing. Your type check narrows x.bar; for this to work, it would also need to narrow x as a whole. If you change the function to return x you’ll see you get the same error.

[MartinJohns]

Just additionally pointing out that the ensureBarIsSet function is completely useless. It's neither a type guard, nor an assertion function. Just a function returning a boolean, a result that is then discarded.

[Akiyamka]

@MartinJohns you right, it's mistake in example