infer extends number in conditional type handles '\n' incorrectly

[yenshih]

Bug Report

🔎 Search Terms

• infer extends number

🕗 Version & Regression Information

4.8.0-beta (in playground)

⏯ Playground Link

Playground link with relevant code

A simple type-level JSON parser encounters this issue

💻 Code

// number
type Foo = '\n' extends `${infer T extends number}${infer R}` ? T : never;

🙁 Actual behavior

Foo is number

🙂 Expected behavior

Foo should be never

[Josh-Cena]

Whitespace has always been valid `${number}`

[yenshih]

It doesn't make sense...
It is definitely counterintuitive :(

[Josh-Cena]

Same feeling. It's a valid JS StringNumericLiteral production regardless, which means if you have Number(" ") it gives you 0 instead of NaN. The inconsistency is that "" is not a valid `${number}` while being a valid StringNumericLiteral.

[MartinJohns]

IIRC you can pass anything that is compatible with parseInt(). There was an issue about this, but I can't find it right now.

[Josh-Cena]

Not just parseInt()—but Number(). It uses the StringNumericLiteral production, which means 0b111 is a `${number}` as well. (Disclaimer: didn't read that source code; this is based on experiment).

[RyanCavanaugh]

@ahejlsberg this seems like wrong behavior - can you confirm?

[fatcerberus]

I would like to point out that due to the consecutive placeholders...

type Foo = '812' extends `${infer T extends number}${infer R}` ? T : never;
//   ^? type Foo = 8

...infer T will only ever match the first digit. There's an issue about this (it's by design) but I can never find it when I need to. It even affects cases like `${number}${string}` where you might expect it to be greedy 🐽

[whzx5byb]

@fatcerberus You are looking for #46124 (comment) ?

[ahejlsberg]

@RyanCavanaugh It does seem like odd behavior, but the expression +s produces 0 when s is just whitespace, and so far we've said that any finite number result from +s is acceptable. I suppose we could additionally validate that s isn't just whitespace.

[dimitropoulos]

by the way, for whoever does this (can I do it? seems really small?) I happened to notice that it's not just empty whitespace you gotta check for, but also whitespace that surrounds (otherwise valid) numbers. So 00 works, as does 00 , 001 , or any hex/octal/binary/scientific notation. You can get silly and add lots of junk:

type Foo = ' \t\n001234e1\r\u0020\u2029 ' extends `${infer T extends number}` ? T : never;
//   ^? number