test-helper.ts

type ProviderPitcherRequest<T extends Provider> = T["onRequest"] extends (
  request: infer Req,
  dispatcher: Dispatcher<any, any>,
  client: Client
) => void
  ? Req
  : never;

type ProviderPitcherResult<T extends Provider> = T["onRequest"] extends (
  request: any,
  dispatcher: Dispatcher<infer Res, any>,
  client: Client
) => void
  ? Res extends { status: PitcherResponseStatus.RESOLVED }
    ? Res
    : never
  : never;

type ProviderPitcherError<T extends Provider> = T["onRequest"] extends (
  request: any,
  dispatcher: Dispatcher<infer Res, any>,
  client: Client
) => void
  ? Res extends { status: PitcherResponseStatus.REJECTED }
    ? Res
    : never
  : never;

type ProviderPitcherNotification<T extends Provider> = T["onRequest"] extends (
  request: any,
  dispatcher: Dispatcher<any, infer Not>,
  client: Client
) => void
  ? Not
  : never;

type DispatchArgs<T> = T extends jest.Mock<any, infer A> ? A : never;

async function onRequest<T extends Provider>(
  instance: T,
  request: ProviderPitcherRequest<T>
): Promise<{
  result: jest.Mock<void, [ProviderPitcherResult<T>]>;
  error: jest.Mock<void, [ProviderPitcherError<T>]>;
  notification: jest.Mock<
    void,
    [string | Set<string>, ProviderPitcherNotification<T>]
  >;
}> {
  return {} as any;
}

async () => {
  const { result, error, notification } = await onRequest(new FS(), {
    method: "fs/createDirectory",
    params: {
      uri: "",
    },
  });

  expect(notification).toHaveBeenCalledWith<DispatchArgs<typeof notification>>('123', {
    method: 'fs/watch',
    
  });
};