mongodb broken by #41821

[sandersn]

mongodb has a mapped type SetFields that calls a conditional type Flatten. Before #41821, Flatten propagated its constraint from its argument to its return type. Now it does not.

To reproduce:

1. npm i mongodb@4.1.2
2. Create a tsconfig with index.ts and strict: true.
3. In index.ts, import mongodb = require("mongodb")
4. Error in mongodb.d.ts on line 5233.

I'll see if I can create a standalone repro.
If this error is intended, we need to provide a fix for mongodb so they don't break when people start using 4.5.

[sandersn]

Here's a standalone repro (I think you still need strict: true):

type Flatten<Type> = Type extends ReadonlyArray<infer Item> ? Item : Type;
type KeysOfAType<TSchema> = {[key in keyof TSchema]: key}[keyof TSchema];

type OptionalId<TSchema extends {_id?: any;}> = TSchema;
type SetFields<TSchema> = {
    readonly [key in KeysOfAType<TSchema>]?: OptionalId<Flatten<TSchema[key]>>;
}

Errors are on Flatten use inside SetFields.

[weswigham]

It may be a regression, but it's 100% correct. We should have always issued an error on this, but failed to due to before because we took too long to expand the conditional type constraints (and issued a bogus Maybe result). OptionalId demands that the type you pass it have an optional _id member. Both of Flatten's branches are only constrained to unknown, and thus aren't proven to have that member. You can work around this with the traditional Cast conditional type if you want to, which would look like type Cast<T, U> = T extends U ? T : T & U; and then write OptionalId<Cast<Flatten<TSchema[key]>, {_id?: any}>>. But better would just be removing the constraint on OptionalId's type argument if it's intended to be used on unknown types like this.

[sandersn]

Fixed by mongodb/node-mongodb-native#3004