Svelte 5: `:nth-child(n+1)` and `:nth-of-type(n+1)` do not compile

[kran6a]

Describe the bug

The bug is self explanatory.

Reproduction

The following code causes the compiler to crash indicating the CSS identifier is invalid.

<style>
	h1:nth-of-type(2n+1){
        background: red;
    }
</style>

<h1>Broken</h1>
<h1>Broken</h1>
<h1>Broken</h1>

Svelte 5 REPL link

The same code compiles and works on Svelte 4 REPL.

Logs

No response

System Info

Not needed

Severity

blocking an upgrade

[gdamjan]

this got broken between 5.0.0-next.17 (works) and 5.0.0-next.18 (fails to compile)

[JitPackJoyride]

i was doing some digging around for the source of the issue, and found that the error is triggered by the code block starting in line 479 in compiler/phases/1-parse/read/style.js:

if (parser.match('--') || parser.match_regex(REGEX_LEADING_HYPHEN_OR_DIGIT)) {
    error(start, 'invalid-css-identifier');
}

i can't tell what should be done because clearly any leading hyphens or digits is not expected, but that is exactly what happens with the (2n + 1) in :nth-of-type(2n + 1).

however, this exact pseudo class worked with previous versions of svelte 5. so i assume it is to do with the logic that got changed in read_selector in line 235 of the same file.

note that i'm referencing the file as of commit fd4a52c