never return type not handled when function has implicit type

🔎 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

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.

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

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never return type not handled when function has implicit type #60609

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🙂 Expected behavior

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never return type not handled when function has implicit type #60609

Description

🔎 Search Terms

🕗 Version & Regression Information

⏯ Playground Link

💻 Code

🙁 Actual behavior

🙂 Expected behavior

Additional information about the issue

Activity

MartinJohns commented on Nov 26, 2024

typescript-bot commented on Nov 29, 2024

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