Inferred return type of functions in array allows incorrect value for optional property of return type

[ulrichstark]

🔎 Search Terms

infer return type function array property optional

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about "return", "property", "properties", "infer"

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.0-beta#code/JYOwLgpgTgZghgYwgAgGLAB7IN4ChnIC2EAziXAOYQD8AXMiWFKBQNy4C+uuYAngA4oAwgAsICANbIAvMgAUAShkA+NJna4EAexCNkCMZJIBGeqPESA2gF0ZyS-nlLpqudg4KANI8Ur52IlJyKnoAVnDkD09kAHoY5AA5AHlkAFEAJXSk9NxrDTjkCAx+ABtgBGAwZCgIMABXKBBkPkFNHT0DCxIAJjNDK1tZBwJfF38onwV6dCwxt0CySggwiKjY+IAVARQAchA6wgAjaB3kYBJkEC0quDJgChA4Q5KUMC1m7eQdxmYQCh3cvl4gB3SoiZAAIlu+naYAhbV0VU6RgAzH0LDY7MMnH43BMRs5XAFiIsQshwqFIl51sgtoIvvsjiczhcrjc7g8ni9mu8WrsfiwAbZodpEUDkKCwCItHUqjBgFA9JUIIQER1+iQACzoySYoaTXHEoJLFaUtYFOm7RnHKCnc6Xa7IW4ke6PZ6vXmfb5MQWA3BAA

💻 Code

interface Fix {
  message?: string;
}

type Check = () => Fix;

const checks: Check[] = [
  () => ({}),
  () => ({ message: 555 }), // no error reported
];

🙁 Actual behavior

No error is reported on the last item of the array.

🙂 Expected behavior

It should report an error, because number 555 is not a legal value for optional property message of type string.

Additional information about the issue

Workaround 1: add explicit return type

const checks: Check[] = [
  () => ({}),
  (): Fix => ({ message: 555 }), // Type 'number' is not assignable to type 'string'
];

Workaround 2: add "as const"

const checks: Check[] = [
  () => ({}),
  () => ({ message: 555 }), // Type 'number' is not assignable to type 'string'
] as const;

Workaround 3: remove first item

const checks: Check[] = [
  () => ({ message: 555 }), // Type 'number' is not assignable to type 'string'
];

[RyanCavanaugh]

Root cause here is #241