Discrepancy inferring widened string type from context

[benjaminjkraft]

Given this code:

declare function compact<T>(array: ArrayLike<T>): T[];
type MyEnum = "a" | "b" | "c";
const myArray = ["a", "b"] as const;
export function f() {
  const _: MyEnum[] = compact([...myArray, "c"]);
}

TS passes but Go gives:

src/compact2.ts:5:9 - error TS2322: Type 'string[]' is not assignable to type 'MyEnum[]'.
  Type 'string' is not assignable to type 'MyEnum'.

5   const _: MyEnum[] = compact([...myArray, "c"]);

i.e. it's inferring the type of the value "c" as string in this context, rather than the type "c" -- which then doesn't typecheck once we get up to the variable declaration.

Possibly the same root cause as #1016, but the details are a bit different (e.g. this one requires the ArrayLike, not just Array).

[ahejlsberg]

This is an odd one. The intended behavior is the error produced by tsgo, the reason being that we don't produce contextual types for array literal elements following a spread element unless the contextual type for the array literal is a true Array<T> or Iterable<T> type (and ArrayLike<T> doesn't qualify). However, due to a bug in tsc (a falsy check that should have been a == undefined check), we produce contextual types if the spread element occurs as the first element of the array literal--but those contextual types are likely to be misaligned with the elements and thus incorrect.

You can witness the inconsistency by the fact that this produces an error (in both tsc and tsgo):

declare function compact<T>(array: ArrayLike<T>): T[];
type MyEnum = "a" | "b" | "c";
const myArray = ["a", "b"] as const;
export function f() {
  const _: MyEnum[] = compact(["a", ...myArray, "c"]);  // Error!
}

So, tsgo is behaving as intended and we have a bug in tsc.

Now, you could argue that we should be smart enough to produce a contextual type from the index signature in ArrayLike<T>, but that's a different discussion and probably not super important.

[Andarist]

There is an old open PR that:

• fixes the mentioned bug
• implements the mentioned contextual typing based on index signatures