In-line chained .then to map an array causes weird behavior with the Map constructor

[didinele]

Bug Report

🔎 Search Terms

map, map constructor, promise, then, await

🕗 Version & Regression Information

• This changed between versions 4.5.5 and 4.6.2

⏯ Playground Link

Playground link with relevant code

💻 Code

interface Data {
    id: string;
}

async function fn(): Promise<Data[]> {
    return [];
}

const map = new Map(
    await fn().then((data) => data.map(d => [d.id, d]))
);

// for top-level-await
export {};

🙁 Actual behavior

map has a type of Map<unknown, unknown>, incorrectly inferring Map<string, Data>.

Interestingly enough, if we were to do

const intermediate = await fn().then((data) => data.map((d): [string, Data] => [d.id, d]));
const map = new Map(intermediate);

the type is inferred correctly. It should also be noted that this issue also doesn't occur when we're not dealing with promises, for example:

function fn(): Data[] {
    return [];
}

const map = new Map(fn().map(d => [d.id, d]);

also works as intended

🙂 Expected behavior

map's type is correctly inferred to Map<string, Data>

[andrewbranch]

The method signature for then is

then<TResult1 = T, TResult2 = never>(
  onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null,
  onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null
): Promise<TResult1 | TResult2>;

What’s happening is the contextual type Iterable<readonly [K, V]> | null from the Map constructor parameter is being used as an inference source for the awaited return type TResult1 | TResult2, so in the end TResult2 gets inferred as Iterable<readonly [unknown, unknown]> which wipes out the correctly inferred result for TResult1 in subtype reduction 😖

[everett1992]

This issue comes up outside map constructor, it can bite you whenever you use .then(onresolve) when the return type is known.

let base: string | undefined

const a = base ?? await Promise.resolve("str").then(val => val)
// Why is this an error - because .then inferred it's return type based on the context
a satisfies string

// See, no error without .then.
const b = base ?? await Promise.resolve("str")
b satisfies string

https://www.typescriptlang.org/play?#code/DYUwLgBARghgziAXBOYBOBLAdgcwgHwgFcsATEAM2xFIFgAoB0SGCAXmnhAgH4eIYAdxgZIABTQB7ALYYEAOjQg4k4ADcQACgBEqNNoCU8sAAsQWTWpjB2APghXgBpuGjtOCXvyEjxU2QpKKupauuiGDAyscDBgclTKKOjYOAxQKLHxGIl6KZH0IAAeAA6SaJAA3gC+QA