never return type not handled when function has implicit type

[jorrit]

🔎 Search Terms

When a function has return type never it can be used as a kind of falsy check:

const test = (input: A|undefined): void => {
  if (input === undefined) {
    neverReturn();
  }

  // input is never undefined here.
  console.log(input.id);
}

This does not work in all cases, as demonstrated on the playground link. Only when the const has an explicit type never is handled correctly.

🕗 Version & Regression Information

It does not work in the any TypeScript version available at the PlayGround.

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=3.8.3#code/JYOwLgpgTgZghgYwgAgILIN4ChnOAEwC5kQBXAWwCNoBuLAXyywQHsQBnMEiAN2gCUIYUlBABGZAF5kACgCUxEL2hSAfJhzIwACygsA7t0MBRKHqgyA5Kkty69OszactEThOkzQAB1JhiqAA+pCD4EDCgEPgKyDwsBGoauMAwsj5+UpLSIWERStFJuNx8UILComLydLiMmgD0dcgAkiC+XAhwIMicwAA2vcjUyDnhkfjIiKxQ+KAA5losyAAqAMoAdJqsHCy9EGu9LLNerX5rBHYMTFsuSiVlIiAATMTyibcqnjHvUInYuDp6QxKExmFgWay2eyOa5cSCcR5SNInfxoYKhUb5GJxBKSdR-PCpY5tTLZdF5KJyQq4b73USPKqaWq4BrNZF4djIMKQKDkMYLQYoEAsLgjcn4Da4a47PYHI7pMBnaL2LBAA

💻 Code

interface A {
  id: number;
}

const neverReturn1 = (): never => {
  throw new Error('A');
};

const test1 = (input: A|undefined): void => {
  if (input === undefined) {
    neverReturn1();
  }

  // Input can still be undefined according to TS.
  console.log(input.id);
}

const neverReturn2: () => never = (): never => {
  throw new Error('A');
};

const test2 = (input: A|undefined): void => {
  if (input === undefined) {
    neverReturn2();
  }

  // Input is determined to be not undefined.
  console.log(input.id);
}

🙁 Actual behavior

Error: Object is possibly 'undefined'. in test1.

🙂 Expected behavior

No errors, because neverReturn1() never returns.

Additional information about the issue

No response

[MartinJohns]

This is working as intended, see #32695. There are so many duplicates of this already. This limitation is for performance reasons.

A function call is analyzed as an assertion call or never-returning call when

• the call occurs as a top-level expression statement, and
• the call specifies a single identifier or a dotted sequence of identifiers for the function name, and
• each identifier in the function name references an entity with an explicit type, and
• the function name resolves to a function type with an asserts return type or an explicit never return type annotation.

---

Your search terms are rather excessive, no wonder you can't find a duplicate like this. I suggest to use a few related words as search terms, often including the modifier in:title. Definitely don't enter entire explanations and even code examples as the search terms.

[typescript-bot]

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