Make typeguards for newtypes match the type instead of isString or isNumber.

server/types/index.ts

@@ -3,6 +3,7 @@ import {
     Domain,
     DomainSpecificNodeResponse,
     EmailAddress,
+    isNumber,
     JSONObject,
     MonitoringResponse,
     MonitoringState,
@@ -13,6 +14,7 @@ import {
     Site,
     StoredNode,
     toIsEnum,
+    toIsNewtype,
 } from "./shared";
 
 export * from "./config";
@@ -90,12 +92,13 @@ export function toMonitoringResponse(node: StoredNode): MonitoringResponse {
     };
 }
 
-// TODO: Complete interface / class declaration.
 export type NodeSecrets = {
     monitoringToken?: MonitoringToken,
 };
 
 export type MailId = number & { readonly __tag: unique symbol };
+export const isMailId = toIsNewtype(isNumber, NaN as MailId);
+
 export type MailData = JSONObject;
 
 export enum MailType {