The new NonNullable type doesn't work the same when combined with index types

[niieani]

Bug Report

🔎 Search Terms

NonNullable index

🕗 Version & Regression Information

• This changed between versions 3.7.4 and 3.8.1-rc

⏯ Playground Link

Playground link with relevant code

💻 Code

type Edge<Typename extends string = string> = {
  readonly events: ReadonlyArray<{ readonly __typename: Typename } | null>;
};

export type InnerEventTypename<EdgeT extends Edge> = NonNullable<EdgeT['events'][number]>['__typename'];

🙁 Actual behavior

Error: Type '"__typename"' cannot be used to index type 'NonNullable<EdgeT["events"][number]>'.

🙂 Expected behavior

No error.

[RyanCavanaugh]

Simplified

type A = {
    prop: { x: string, y: string } | null
};
// 4.7: OK
// 4.8: Error
type Test<T extends A> = NonNullable<T['prop']>['x'];

[ahejlsberg]

This was an issue only in 4.8.0-beta. It was fixed in 4.8.1 and doesn't currently repro.

[RyanCavanaugh]

I oversimplified, then. This is (newly) erroring in 4.8.2

type Column<T = unknown> = (keyof T extends never ? { id?: number | string } : { id: T }) & {
    title?: string;
}

// no error if `key: keyof Column`
function getColumnProperty<T>(column: Column<T>, key: keyof Column<T>) {
    return column[key];
}

[RyanCavanaugh]

Actually since OP is fixed, let's track over at that one - #50465