Matcher type in Pathname

[WAKayser]

Describe the problem

I have a site with a optional matcher for a language id at the start of the path.. This allows me to translate into a predefined set of languages. But the type for Pathname just resolves to string at that url part. So for the / path this add | `${string}` & {}; to the type. Which causes any string to be accepted for pathname.

For the accepted languages I have a well defined list of ids. Similar to the matching example in the docs: https://svelte.dev/docs/kit/advanced-routing#Matching.

Describe the proposed solution

Ideally we could figure out the type and use that instead of string.

This could be a type exported from the matcher file. For example:

import type { ParamMatcher } from '@sveltejs/kit';

export type Param = 'apple' | 'orange';

export const match = ((param: string): param is Param => {
	return param === 'apple' || param === 'orange';
}) satisfies ParamMatcher;

this is the more explicit version of defining the Param. but it can also be done by inspecting the match.

type ExtractPredicateType<T> = T extends (param: string) => param is infer U ? U: never;

export const match = ((param: string): param is (typeof locales)[number] => {
	return isValidLocale(param);
}) satisfies ParamMatcher;

type expectedValues = ExtractPredicateType<typeof match>;

Alternatives considered

No response

Importance

nice to have

Additional Information

No response