4.1.1-rc strange Type is not assignable to type never with yield & generators #41428

const selection = yield step;

export enum Directive {
	Back,
	Cancel,
	LoadMore,
	Noop,
}

export namespace Directive {
	export function is<T>(value: Directive | T): value is Directive {
		return typeof value === 'number' && Directive[value] != null;
	}
}

export interface QuickPickItem {
	label: string;
	description?: string;
	detail?: string;
	picked?: boolean;
	alwaysShow?: boolean;
}


export interface QuickInputStep {

	placeholder?: string;
	prompt?: string;
	title?: string;
}

export interface QuickPickStep<T extends QuickPickItem = QuickPickItem> {
	placeholder?: string;
	title?: string;
}

export type StepGenerator =
	| Generator<QuickPickStep | QuickInputStep, StepResult<void | undefined>, any | undefined>
	| AsyncGenerator<QuickPickStep | QuickInputStep, StepResult<void | undefined>, any | undefined>;

export type StepItemType<T> = T extends QuickPickStep<infer U> ? U[] : T extends QuickInputStep ? string : never;
export namespace StepResult {
	export const Break = Symbol('BreakStep');
}
export type StepResult<T> = typeof StepResult.Break | T;
export type StepResultGenerator<T> =
	| Generator<QuickPickStep | QuickInputStep, StepResult<T>, any | undefined>
	| AsyncGenerator<QuickPickStep | QuickInputStep, StepResult<T>, any | undefined>;
export type StepSelection<T> = T extends QuickPickStep<infer U>
	? U[] | Directive
	: T extends QuickInputStep
	? string | Directive
	: never;
export type PartialStepState<T = unknown> = Partial<T> & { counter: number; confirm?: boolean; startingStep?: number };
export type StepState<T = Record<string, unknown>> = T & { counter: number; confirm?: boolean; startingStep?: number };

export function canPickStepContinue<T extends QuickPickStep>(
	_step: T,
	_state: PartialStepState,
	_selection: StepItemType<T> | Directive,
): _selection is StepItemType<T> {
	return false;
}

export function createPickStep<T extends QuickPickItem>(step: QuickPickStep<T>): QuickPickStep<T> {
	return step;
}

export function* showStep<
	State extends PartialStepState & { repo: any },
	Context extends { repos: any[]; title: string; status: any }
>(state: State, _context: Context): StepResultGenerator<QuickPickItem> {
	const step: QuickPickStep<QuickPickItem> = createPickStep<QuickPickItem>({
		title: '',
		placeholder: ''
	});
	const selection: StepSelection<typeof step> = yield step;
	return canPickStepContinue(step, state, selection) ? selection[0] : StepResult.Break;
}