narrow [nfc]: Have specialNarrow take string-literals union, not string

Oddly, the regex was doing a looser check on the operand than
`specialNarrow` was: it was case-insensitive, and it was only
looking at the beginning of the string. That doesn't end up making a
difference, because `specialNarrow` would throw an error and we'd
return null, the same as if the regex hadn't matched the string.

Author: chrisbobbe

src/utils/internalLinks.js

@@ -173,13 +173,11 @@ export const getNarrowFromNarrowLink = (
   if (
     hashSegments.length === 2
     && hashSegments[0] === 'is'
-    && /^(private|starred|mentioned)/i.test(hashSegments[1])
+    && (hashSegments[1] === 'starred'
+      || hashSegments[1] === 'mentioned'
+      || hashSegments[1] === 'private')
   ) {
-    try {
-      return specialNarrow(hashSegments[1]);
-    } catch {
-      return null;
-    }
+    return specialNarrow(hashSegments[1]);
   }
 
   return null; // TODO(?) Give HOME_NARROW

src/utils/narrow.js

@@ -13,6 +13,7 @@ import {
   makePmKeyRecipients_UNSAFE,
 } from './recipient';
 import type { UserMessageFlag } from '../api/modelTypes';
+import { ensureUnreachable } from '../generics';
 
 /* eslint-disable no-use-before-define */
 
@@ -123,17 +124,18 @@ export const pmNarrowFromUsersUnsafe = (recipients: $ReadOnlyArray<UserOrBot>):
 export const pm1to1NarrowFromUser = (user: UserOrBot): Narrow =>
   pmNarrowInternal(pmKeyRecipientsFor1to1(user.user_id));
 
-export const specialNarrow = (operand: string): Narrow => {
-  if (operand === 'starred') {
-    return Object.freeze({ type: 'starred' });
-  }
-  if (operand === 'mentioned') {
-    return Object.freeze({ type: 'mentioned' });
-  }
-  if (operand === 'private') {
-    return Object.freeze({ type: 'all-pm' });
+export const specialNarrow = (operand: 'starred' | 'mentioned' | 'private'): Narrow => {
+  switch (operand) {
+    case 'starred':
+      return Object.freeze({ type: 'starred' });
+    case 'mentioned':
+      return Object.freeze({ type: 'mentioned' });
+    case 'private':
+      return Object.freeze({ type: 'all-pm' });
+    default:
+      ensureUnreachable(operand);
+      throw new Error();
   }
-  throw new Error(`specialNarrow: got unsupported operand: ${operand}`);
 };
 
 export const STARRED_NARROW: Narrow = specialNarrow('starred');