String#matchAll is too restrictive in its parameter type

[bbrk24]

🔎 Search Terms

matchAll

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about RegExps and matchAll

⏯ Playground Link

https://www.typescriptlang.org/play?target=7#code/MYewdgzgLgBAtgTwMIFdojgeQEYCsCmwsAvDAN4CwAUDLTAFQDaAygnNiADYB0cAhlGAALAIKdOAXQAU0AE4BLMAHMAXDDmKlASjUBxfGHyyBIWQB4ASviUBRAB4AHe4RGzjCAHzlqdGAF9qAKpqUEgufG5OECUpRm54gHIAMxAQBN4BYTFOKURUdCw8QigtCS0AbmogA

💻 Code

const myCustomObject = {
    *[Symbol.matchAll](string: string): Generator<RegExpExecArray> {
    }
}

console.log([...'foo'.matchAll(myCustomObject)]);

🙁 Actual behavior

TS rejects this code, because myCustomObject is not an instance of RegExp.

🙂 Expected behavior

Any value matching the following type should be accepted, including myCustomObject. The other parts of the RegExp object are not used.

type MatchAllRegExpLike = {
  [Symbol.matchAll](string: string): IterableIterator<RegExpExecArray>;
  // If the object has a [Symbol.match] property that is truthy, its flags are checked. However, neither
  // must be present, and even if it is present, [Symbol.match] doesn't have to be a function.
  readonly [Symbol.match]?: any;
  readonly flags?: string;
};

Additional information about the issue

This is distinct from #47310. That issue is about string-to-regexp conversion; this is about regexp-like objects.

[lionel-rowe]

Same issue also exists for Symbol.match, Symbol.replace, Symbol.search, Symbol.split and their corresponding string methods. edit: nvm looks like this is fixed now.

The actual (runtime) types are pretty lenient, but deterministic. E.g. for matchAll, it's really more like this:

type MatchAllArgument<T> = {
    [Symbol.matchAll](string: string): T
} & (
    ({ [Symbol.match]?: never })
    | { [Symbol.match]: unknown; flags: string }
)

interface String {
    matchAll<T>(pattern: MatchAllArgument<T>): T
}

And you could easily define a matcher like this where T is number or something crazy:

const matcher = {
    [Symbol.matchAll]: (str) => str.length
}

const x: number = 'abc'.matchAll(matcher)