Spreading variable of `RegExpIndicesArray` type will cause error

[hkleungai]

Steps to reproduce

1. Start a plain new project, with @typescript/native-preview@npm:7.0.0-dev.20260211.1 as dependency.
2. Run npx tsgo --init to get the default tsconfig.
3. Add a src/main.ts & run npx tsgo --noEmit.

// src/main.ts

declare let indices: RegExpIndicesArray;
let [[x, y]] = indices;

Behavior with typescript@6.0

No error, as it should be. See https://tsplay.dev/mqb9ZW.

Behavior with tsgo

src/main.ts:4:6 - error TS2488: Type 'undefined' must have a '[Symbol.iterator]()' method that returns an iterator.

4 let [[x, y]] = indices;
       ~~


Found 1 error in src/main.ts:4

---

Note: The error is the same as if indices is of type indices | undefined. It seems only happen to RegExpIndicesArray, so I wonder if it is the case that tsgo cannot infer extended array element type correctly during spreading.

[ahejlsberg]

The RegExpIndicesArray interface is declared in lib.es2022.regexp.d.ts as

interface RegExpIndicesArray extends Array<[number, number] | undefined> {
    groups?: {
        [key: string]: [number, number];
    };
}

so the error is expected (because elements might be undefined). Both tsc and tsgo report the error. But oddly, there's no error in the playground, and the element type appears to just be [number, number], not [number, number] | undefined. I have no idea what is going on there.

[jakebailey]

Have you seen #2768? It seems to resolve this by porting over more code, and fixes other diffs...

[ahejlsberg]

Right, but that just changes the error reporting. It doesn't make the error go away. And I'm not understanding why there is no error in the playground when an error is reported by tsc.

[ahejlsberg]

And, TBH, I like the error reporting in tsgo better because it exactly tells you which type isn't iterable.

[hkleungai]

A further note: If I do a non-null assertion on indices on the line of spreading, I can still see the error in tsgo (but not in tsc).

declare let indices: RegExpIndicesArray;
let [[x, y]] = indices!;

Looks like it is Array<undefined> instead of undefined, non-null assertion will not (and should not) work in this case.

[hkleungai]

@ahejlsberg May I know which version of tsc you are referring to? I have 5.9.3 in my local, and the above code passes tsc typecheck.

[jakebailey]

You said 6.0 in your issue description; we're targeting 6.0 nightly, not 5.9.3.

[hkleungai]

Ohh, I see it now. I guess this issue becomes two (smaller) issues then.

1. Error reporting.
   • tsc v6 reports Type '[number, number] | undefined' must have a '[Symbol.iterator]()' method that returns an iterator.
   • but tsgo reports Type 'undefined' must have a '[Symbol.iterator]()' method that returns an iterator.
2. Typescript playground behaviour is inconsistent with the actual tsc cli output at v6.

[jakebailey]

I think there's an issue with the playground libs in general after we added es2025 and so on. I'll go try and fix it.

[hkleungai]

I am also a bit surprised to learn that type inference behaviour on RegExpIndicesArray is changed between 5.9 & 6.0. Is it yet another change for TS consumer to discover on their own? Because, it looks like 6.0 beta release note has not covered it.

[jakebailey]

The release notes can't cover every small detail; this is likely a bugfix that happened somewhere else. You could try using every-ts to bisect it.