Discriminated unions is not work when field key is number

🔎 Search Terms

• discriminated unions, "discriminated-unions"
• "numbers as keys in object", "number key"
• "type guard"

🕗 Version & Regression Information

• This is a crash

⏯ Playground Link

https://www.typescriptlang.org/play?#code/C4TwDgpgBAglC8UB2BXAtgIwgJwNoF0oAfKAZ2GwEskBzAgKHtEigCEEoBvKABgC5k6LNgA0UAIYCAjFAC+xLrwHkqtMZKgByKZrmMAJhADGAG3HZoRgPZJyEgTHqHT5yzbsYBrRpQBmUAApmCCt-cVweQnhorVRMHE0ASi56KDSoa1srEwgAOhMrGgDxRNT0gHpy9Or0gD0AfnpZej9A4NCoDFxNcU0omM044SSU6szSbLyCooxS6sqamoamlv8g8BD-Ls0ePoQBoYTkzjK08cn8woDZ06gFxbrG5tb1yA6uyP3EQaEj0fTzjlLjM5hUqg80stmkA

💻 Code

type A = number[] | string[]

type B = { 0: number, a: 1 } | { 0: string, a: '1' }

declare const a: A
declare const b: B

if (typeof a[0] === 'number') {
    console.log(a)
    //          ^? A
}
if (typeof b['a'] === 'number') {
    console.log(b)
    //          ^? { 0: number, a: 1 }
}
if (typeof b['0'] === 'number') {
    console.log(b)
    //          ^? B
}
if (typeof b[0] === 'number') {
    console.log(b)
    //          ^? B
}

🙁 Actual behavior

Unable to type guard b as an option of the union type B.

🙂 Expected behavior

Type b as { 0: number, a: 1 } when type guarding it with typeof b['0'] === 'number'.

Additional information about the issue

• microsoft/TypeScript#55632
• microsoft/TypeScript#55766
• microsoft/TypeScript#55961

Start with the linked TypeScript Playground example and compare the existing behavior for string and numeric keys. Read the related issues microsoft/TypeScript#55632, #55766, and #55961 to understand the type-guard context. Done means typeof b['0'] === 'number' narrows B to { 0: number, a: 1 } without breaking the other examples.