Promise.all on any produces Promise<Record<string, any>>

[DanielRosenwasser]

Found in a large internal codebase:

async function foo(x: any) {
    let abc = await Promise.all(x);
    let oops: any[] = abc;
//      ~~~~
// error: Type '{ [x: string]: any; }' is missing the following properties from type 'any[]': length, pop, push, concat, and 26 more.
}

Promise.all always produces an Array, so this behavior is undesirable.

What's happening if you can't tell is that any is a valid substitution for T extends unknown[]. Upon instantiation, we feed any through a mapped type which acts differently depending on if we've passed in an array or something else.

[DanielRosenwasser]

When instantiating with any, we could instead choose an array-like type when we encounter a deferred homomorphic mapped type that's bounded by an array-like type.

In other words Foo<any> would produce different results (e.g. any[]) compared to than Bar<any> (i.e. Record<string, any>) here:

type Foo<T extends unknown[]> = { [K in keyof T]: T[K] };

type Bar<T extends unknown> = { [K in keyof T]: T[K] };

That wouldn't always work, and could result in behavior that progressively changes every time we instantiate a mapped object type, but I think that's probably usually desirable anyway, and it would work here.

[DanielRosenwasser]

Probably something similar to this in instantiateMappedType

const constraint = getConstraintOfTypeParameter(typeVariable);
if ((t.flags & TypeFlags.Any) && (isArrayType(constraint) || isTupleType(constraint))) {
    return instantiateMappedArrayType(t, type, prependTypeMapping(typeVariable, t, mapper));
}

[DanielRosenwasser]

Seems from our new errors report that TypeORM has two regressions from the Promise.all case:

• https://github.com/typeorm/typeorm/blob/861573377bb33b73232399c21b1b3a5c07b58036/sample/sample25-insert-from-inverse-side/app.ts#L48
• https://github.com/typeorm/typeorm/blob/861573377bb33b73232399c21b1b3a5c07b58036/src/entity-manager/MongoEntityManager.ts#L118