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Cannot assert about union type of assertions #62513

Closed as not planned
Closed as not planned
Cannot assert about union type of assertions#62513
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Design LimitationConstraints of the existing architecture prevent this from being fixed

Description

@nnnnoel
Noel Kim (nnnnoel)
opened on Sep 30, 2025

🔎 Search Terms

union type of type assertion

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/GYVwdgxgLglg9mABAQwM6oKYCcqoIwAUAHgFyKpRYxgDmiAPomCALYBG2Di4AJhsNQw8AlGTSYcqREUQBvAL4BYAFChIsBCnTZcAJmJkKVWl179BIsdsnS5S5SogIKiFsgCeHAMqVqdALyIALLIUAAWAHRYyGA8cCwEwogAfIgADBEArIgA-IgA5EZ++RGoADYwEBiJiGRmAmBCANwqjs5Q5PEYAILWUGRQ7gAOGHDAWhK4eFyDI2MTOqi6iP4qiMGhkdGx8TWpGdl54ovTVpNLLQ6q4NDwSMg1smvrnSw9fQRunhg+xjTClxerg83l8tAiEDCyCw3SgBDSAJU9iAA

💻 Code

function asserts1(x: string | number | undefined): asserts x {}
function asserts2(x: string | undefined): asserts x {}

const maybeString = Math.random() > 0.5 ? 'string'.slice() : undefined;

const someAssert: typeof asserts1 | typeof asserts2 =
  Math.random() > 0.5 ? asserts1 : asserts2;

function a() {
    someAssert(maybeString);
    maybeString.charAt(0);
}

🙁 Actual behavior

maybeString is string | undefined

🙂 Expected behavior

maybeString must have only string type

Additional information about the issue

Related: #59707

When i implementing lint rule for about assert functions, cannot be report error coz checker doesn't respect someAssert is assertion function.
oxc-project/tsgolint#222
https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts#L3547

Activity

  1. MartinJohns commented on Sep 30, 2025

    @MartinJohns
    Martin Johns (MartinJohns)
    on Sep 30, 2025
    Contributor

    This is working as intended, as per #32695.

    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.

    The last point is not fulfilled, as your variable resolves to a union type.

  2. Ryan Cavanaugh (RyanCavanaugh)
    added
    Design LimitationConstraints of the existing architecture prevent this from being fixed
    on Sep 30, 2025

  3. typescript-bot commented on Oct 3, 2025

    @typescript-bot
    TypeScript Bot (typescript-bot)
    on Oct 3, 2025
    Contributor

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

  4. TypeScript Bot (typescript-bot)
    closed this as not plannedon Oct 3, 2025
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