Broken emit when `Infinity` or `‑Infinity` ends up in a type position

Bug Report

🔎 Search Terms

• Infinity
• NaN
• numeric
• literal

🕗 Version & Regression Information

• Tested in: TypeScript 4.1.5 and TypeScript 4.3.0‑dev
• I was unable to test this on prior versions because https://github.com/microsoft/TypeScript/pull/9407 was implemented in TypeScript 2.0.

💻 Code

// @declaration
// @showEmit
// @showEmittedFile: index.d.ts

// @filename: index.ts
export const PositiveInfinity: 1e1_000_000 = 1/0 as any;
export const NegativeInfinity: -1e1_000_000 = -1/0 as any;

⏯ Playground Link

Workbench Repro

🙁 Actual behavior

TypeScript emits Infinity and ‑Infinity in a type position, which are intentionally invalid according to https://github.com/microsoft/TypeScript/pull/9407#issuecomment-229721835:

// index.d.ts
export const PositiveInfinity: Infinity;
export const NegativeInfinity: -Infinity;

🙂 Expected behavior

The Infinity and ‑Infinity values are valid in a type position, so that the generated .d.ts file is valid.

---

Also, it’d be nice to support literal NaNs, which would allow for Number.isNaN to be typed as:

interface NumberConstructor {
	isNaN(number: unknown): number is NaN;
}

[Jamesernator]

In your example you could just as well replace the Infinity with undefined. Having a dedicated Infinity type would not help you in this case, because Infinity would be a sub-type of number, so the union type number | Infinity would be simplified to number.

The example is bigint not number. BigInts don’t contain an Infinity value, I think it’s a lot more readable to use the explicit Infinity value rather than some arbitrary token as it already compares correctly with bigints (e.g. bigint < Infinity is always true, no coercion to Number happens).

[MartinJohns]

You’re mixing a bug report and a feature request together in one. Fixing the bug would be to emit the type number instead of Infinity. Introducing an Infinity (and NaN) type is a feature request, for which you should provide a good reasoning in response to https://github.com/microsoft/TypeScript/pull/9407#issuecomment-229721835.