Allow Inifinity and -Infinity as number literal types

Please accept Infinity and -Infinity as valid number literal types.

/** A range of BigInt that may be unbounded, e.g., (-∞, 5] */
class BigIntRange {
  public min: bigint | -Infinity = 0n; // <= Error!
  public max: bigint | Infinity = 1n; // <= Error!
  public isValid(): boolean {
    return this.min <= this.max;
  }
}

Currently, This causes a compile error, 'Infinity' refers to a value, but is being used as a type here. (2749).

I understand this was once rejected as part of #15135 and #15356, and the given reasons were: 1) difficult to implement, and 2) lacks a compelling use case.

However, while I understand why NaN as a literal type is tricky to implement, NaN and Infinity are not the same. And I have a clear use case for Infinity as a meaningful literal type, as shown above.

Infinity is not tricky to compare

Unlike notorious NaN or -0, Infinity works just like an ordinary numeric constant as far as equality is concerned. We don't need a special function like isNaN() or Object.is(). Ever since the ES5/IE6 era, there has been nothing counter-intuitive:

Infinity === Infinity // => true
-Infinity === -Infinity // => true
Number.POSITIVE_INFINITY === Infinity // => true
Object.is(-Infinity, Number.NEGATIVE_INFINITY) // => true
10 ** 500 === Infinity // => true (because 10**500 is above Number.MAX_VALUE)

typeof Infinity === 'number' // => true
typeof Number.NEGATIVE_INFINITY === 'number' // => true

// FWIW, comparison works just as expected, too
50 < Infinity // => true
-Infinity < 50; // => true
10n ** 500n < Infinity // => true
Number.MAX_VALUE < Infinity // => true
Infinity < Infinity // => false

Most importantly, Infinity === Infinity is true (while NaN === NaN is false). Unless I'm missing something, predictable equality is all that's required to safely use Infinity as a literal type, right? Even though the design note (#15356) says "there is more than one NaN, Infinity, etc", you can think of Infinity in JavaScript as "just a fixed number" which happens to be larger than Number.MAX_VALUE.

My use case

My library deals with unbounded (aka infinite) integer ranges, and I have been using Infinity and -Infinity without any issue to denote what they literally mean, infinity in the mathematical sense. I have instructed my users to use these interesting constants, too. Recently I started to extend my library to support bigint in addition to number, and ran into this problem.

You may ask "Why don't you just use string 'Infinity' or Symbol('unbounded')", but Infinity is a predefined and predictable constant, and it can be directly compared with any bigint (e.g., 10n ** 1000n < Infinity is true). See how simple the implementation of isValid can be in the first example.

PS: Looks like there is a hacky workaround (#31752), but I'd like to see the official support.

この issue では実装ファイルやテストが指定されていないため、まず #15135 と #15356 の過去の議論、および #31752 の workaround を確認してください。コンパイラが Infinity と -Infinity を数値リテラル型として、BigIntRange の例も含めて、報告された型エラーなしに受け入れれば完了です。