🔎 Search Terms

narrow, generic

🕗 Version & Regression Information

• This changed between versions 5.3.3 and 5.4.2

⏯ Playground Link

https://www.typescriptlang.org/play?#code/PTAEF5NB1B5AlA0gZQpdH0CgsBMCmAxgDYCGATvqAGYCuAdoQC4CWA9vaCwM4AKZLerABGAKyJMAjAAosoULlJNSALlAMA1vTYB3egBosASjWLlXbqHhE25XAB5e5NgAd85JgE9E+T-vX0Wrr0AHwA3DgEJBRUhBzcTKAAtp4iopJqmtp6ESzUoNI8-KSCaRIyKWmSRkagAN5y8jS2BXH0CaAavqBs+QDaAESkA-4DwiOgA4QDALq1DU1NbR0E+G7kEMmpYpJ9XZ4zEYvyIIsAegD8jYvLiavrPp6W4BbFpWLl0vfutRegZcwAHT7bhffBrH6gNR9Q7XUAAXywiKwp3QoAAIrAAKLIAByABUYAhEGhMGTsKdYPRiJ4FHlqO58IwqMJ8EwdODOEwABZUABMXCSLmIlh4oB5VAA5kz3CxCKBCYJxbzxZ43P58AA3JmgHS8zgsRI8egAckStG4+FwgJtkSIZEoNAYzHYBr4AiEH2YfPs+JCsnkZlUASCekMJgUSlIFisNjsjmc6y8j38WWC4Tt0Udty2aT5mUC2XouXyhXdJU94m90kqYj5NXqcOoLWkOf2PX6QwmY270zmjeOoBz3w2L1roj5e18sMHpyalzh8mH4Ievmerw9AKYfLBEPIv3+XqYwLXu-WtWhM6aiMRQA

💻 Code

// === WORKS =============

declare function isPlainObject1(
  data: unknown,
): data is Record<PropertyKey, unknown>;

declare const myObj1: unknown;
if (isPlainObject1(myObj1)) {
    for (const key of ["a", "b", "c"]) {
      const deeper = myObj1[key];
      //    ^?
      const deeperKeys = isPlainObject1(deeper) ? Object.keys(deeper) : [];
    }
}

// === DOESNT WORK ================
// Only difference between the 2 impls is the generic T in the type, even when it isn't used...

declare function isPlainObject2<T>(
  data: unknown,
): data is Record<PropertyKey, unknown>;

declare const myObj2: unknown;
if (isPlainObject2(myObj2)) {
    for (const key of ["a", "b", "c"]) {
      const deeper = myObj2[key];
      //    ^?
      const deeperKeys = isPlainObject2(deeper) ? Object.keys(deeper) : [];
    }
}

🙁 Actual behavior

The deeper variable, which is nested deeper from the narrowed type is inferred as any.

🙂 Expected behavior

It is the result of accessing a prop on a Record<PropertyKey, unknown> so is expected to be unknown.

Additional information about the issue

I maintain the Remeda utility functions library, I just got this reported here: remeda/remeda#559 and there are additional anecdotal reports that upgrading to typescript 5.4 breaks several of our types.

I couldn't find a reasoning to this issue, including any new changes to Typescript in 5.4 which might explain it.

I'm not sure my title makes sense for this bug, as I don't fully understand what's going on here, or even how to build a truly minimal example.