New error in 3.9: TS 7053 - can't index with number

[amcasey]

TypeScript Version: 3.9.0-dev.20200420

Search Terms:

Intersection, Union, 7053

Code

export type TypedArray = Int32Array | Uint8Array;

export function isTypedArray(a: {}): a is Int32Array | Uint8Array {
  return a instanceof Int32Array || a instanceof Uint8Array;
}

export function flatten<T extends number|TypedArray>(arr: T) {
  if (isTypedArray(arr)) {
      arr[1]; // TS 7053
  }
}

3.8 behavior: No error
3.9 behavior: Error

Playground Link: https://www.typescriptlang.org/play/?ts=3.9.0-dev.20200420&ssl=1&ssc=1&pln=11&pc=2#code/KYDwDg9gTgLgBDAnmYcAqzgBMCCUoCGicAvHAJIB2MAzAEx6HEA+cAqgJbUAcjRA3AChBoSLDgAzAK6UAxjA4RKcDgGcMKXPiIAKAgC44AbwC+ASkMEVqitXp8W7LjF7biRwXDhRgMKVGUrLlUYAjlgCAlbWgY3OGZWIMoQsNkIqM4eByETYVFoeGk5BSVJABsCGBhgSgAeNDhQasosG0opAFsAI2AoZg1sBwA+PXxDNDNjTxUonTUBrSZRqDNJjy8NuAJ8AG0ARgBdIS9ckyA

Related Issues: I'm guessing this is another empty intersection bug, but I wasn't sure how to de-dup it.

Inspiration is from https://github.com/tensorflow/tfjs/blob/95995e7fec478e86b4baf0d34f902d3e345ee54b/tfjs-core/src/util.ts#L144

[amcasey]

FYI @ahejlsberg

[amcasey]

The error text is

Element implicitly has an 'any' type because expression of type '1' can't be used to index type '(T & Int32Array) | (T & Uint8Array)'.
  Property '1' does not exist on type '(T & Int32Array) | (T & Uint8Array)'.(7053)

[RyanCavanaugh]

@ahejlsberg not sure why this changed; I don't recall any changes around indexing. Observations:

• The narrowed type of arr is the same in 3.9 and 3.8.3
• arr[1] has type number in 3.8.3
• Changing to arr[1 as number] doesn't fix it (just produces an error about no numeric index signature instead)

[ahejlsberg]

It appears to have something to do with discriminants and intersections. Simplified example:

interface NumList {
  kind: 'n';
  [x: number]: number;
}
interface StrList {
  kind: 's';
  [x: number]: string;
}

export function foo<T extends NumList | StrList>(arr: T & NumList | T & StrList) {
  let zz = arr[1];  // Error
}

Error goes away without the kind property or when it is changed to something that isn't a discriminant.

[DanielRosenwasser]

Didn't @orta look into a similar bug?