numeric index type should not merge

[nnnnoel]

🔎 Search Terms

tuple to discriminated union

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about `Common "Bugs" That Aren't Bugs

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.8.0-dev.20250130#code/FAFwngDgpgBAcgVwLZQE4EsDGBlEGB2A5jALwwAGAJAN77IBGaAvuQNyiSwAqA9oko1QAeLjCgAPEFHwATAM4w5edEQB8pGKIlTZCqtRUAzNDACqYydPkw6A5uRgB+MzABcNqADc07YBIg8qCAw4NAwuFAQALI8MlAANgCMIhY61qhQAIYyPPjxYDAA2giyUIYqUDIANDAAdPUlANb4PADu+IUAup3qZNTAMEUA0jAqMACikqiZmCBCjVBgPIaaNbaCqp3u-YODSpHuQ+y7MEixCe5chUOdxzBM7EyFk3gzcwtLK1xrDGibvv5AsFQrAItFzvEAEwpbRWBQZbK5fJFEpxcr4So1eq1JotdpdHoaHbDUb4CZTN7zRbLVbwZBoLC4AiETbbAa7fYQS58X7CIaqO6DM5xeKXQq8fiCIT827sh7AJ4vaazKmfWn8Bk4ZRqWUcMIxEWJDRgg0JZLFUrozEwagwTyZeIIKDudYmJg9dgAek9uwAeo5gHrYKaocapOCRdCLWiKtUbXaHU6Xbz7h7gN6-Y4gA

💻 Code

type NumericString = `${number}`;
type ToNumber<T extends string> = T extends `${infer U extends number}` ? U : never;

export type StepModel1<T extends readonly [undefined, ...unknown[]]> = {
  [K in Extract<keyof T, number>]: {
    step: K;
    model: T[K];
  };
}[Extract<keyof T, number>];

export type StepModel2<T extends readonly [undefined, ...unknown[]]> = {
  [K in Extract<keyof T, NumericString>]: {
    step: ToNumber<K>;
    model: T[ToNumber<K>];
  };
}[Extract<keyof T, NumericString>];

type Model1 = StepModel1<[undefined, { value: number }]>;
//   ^?

type Model2 = StepModel2<[undefined, { value: number }]>;
//   ^?

🙁 Actual behavior

Model1 union types are merged as number type

🙂 Expected behavior

Model1 type also be same with Model2

Additional information about the issue

No response

[jcalz]

Basically a duplicate of #55762

This is working as intended. Yes, it's weird that the known numeric indices of tuples are string literal types and not numeric literal types, but it's a well-established weirdness that's been around for a long time. It's not a bug, anyway.