Strange intersection/variance behavior

TypeScript Version: Nightly (d6dda230bce0828144e5f17539d6e3f9bf4c4f3b)

Search Terms: variance, intersection

Code

interface BaseEvent {
    type: string;
}
interface DerivedEvent extends BaseEvent {
    type: "mytype";
    payload: number;
};
type TransitionConfig<TEvent extends BaseEvent> = (event: TEvent) => void;
type TransitionsConfigMap<TEvent extends BaseEvent> =
    & { splendiferous?: never } // Un/comment this line
    & { [K in TEvent['type']]?: TransitionConfig<TEvent>; };
type TransitionsConfigArray<TEvent extends BaseEvent> = {
    [K in TEvent['type']]: TransitionConfig<TEvent extends { type: K; } ? TEvent : never>;
}[TEvent['type']];
type AssignAction<TEvent extends BaseEvent> = (meta: TransitionsConfigMap<TEvent> | TransitionsConfigArray<TEvent>) => void;
declare const d1: AssignAction<DerivedEvent>;
declare const b1: AssignAction<BaseEvent>;
const d2: typeof d1 = b1; // Sometimes an error
const b2: typeof b1 = d1; // Sometimes an error

As-is, there are errors on both d2 and b2. If you comment out the "splendiferous" line, the error on b2 goes away. (If you disable strict function types, the error on d2 also goes away.) I find this strange, since that line does not reference TEvent.

The type names give breadcrumbs back into xstate, but I've inlined and deleted so many things that the structure is substantially different.

Playground Link: https://www.typescriptlang.org/play?ts=4.0.2#code/JYOwLgpgTgZghgYwgAgEJwM4QKIDcLjIDeAUMucmAJ4AOEAXMhmFKAOYDcJAviaJLEQoAItGD4AJngJhkEAB6QQEjGkw58hUhUq0GyAEQBbKtToGuOmnCoAbAPZwJjEAFcjAI2hduXMygAVKDgQDGAwYHsQAGEomGA2AB4A6UIFJRU1LFSwAD5kAF5kAAoITTBGFPKASkL83HtgCT89ZCCQsIiojFiQeLYAWTgaZJy5RQJM9Gzy-IKyCgAyYiYaW0ngGGh7VwwAfhcy6GReHWWiZABtAGlkUDacy4ByfyeAXTeDtuDQ8MiYuIJUazDgnFp0b4dP7dXr9ACCUGCVGBMnGGVU0w0MjmxAW5BudxAD3Kz1eH0qP06-1hQKqqPSk1UF38jGuoO4yD2xNRh3wUFyPkudPApL07ze4JQcIwYTYIDhCC6IBRaQmygx6hyOOKRggYDgFKhSp6gMGwxVeWQAB9Ib9jTS2AikRbcrUCvVGs0SBIIAhbHAoCgEN1ZBIAIyMaWy+WK-6JUSsSRarg+v0BoMh5AeCPIKMJGNKxKY5MkYOhUMAJkY-nsMGQ4cKWbDXDLzCzVd0dFrTcb4Y4QA