Provide a more detailed error message than "No known servers"

src/i18n/strings/en_EN.json

@@ -878,6 +878,8 @@
     "Please contact your homeserver administrator.": "Please contact your homeserver administrator.",
     "The person who invited you has already left.": "The person who invited you has already left.",
     "The person who invited you has already left, or their server is offline.": "The person who invited you has already left, or their server is offline.",
+    "You attempted to join using a room ID without providing a list of servers to join through. Room IDs are internal identifiers and cannot be used to join a room without additional information.": "You attempted to join using a room ID without providing a list of servers to join through. Room IDs are internal identifiers and cannot be used to join a room without additional information.",
+    "If you know a room address, try joining through that instead.": "If you know a room address, try joining through that instead.",
     "Failed to join": "Failed to join",
     "Connection lost": "Connection lost",
     "You were disconnected from the call. (Error: %(message)s)": "You were disconnected from the call. (Error: %(message)s)",

src/stores/RoomViewStore.tsx

@@ -593,7 +593,7 @@ export class RoomViewStore extends EventEmitter {
             );
         } else if (err.httpStatus === 404) {
             const invitingUserId = this.getInvitingUserId(roomId);
-            // only provide a better error message for invites
+            // provide a better error message for invites
             if (invitingUserId) {
                 // if the inviting user is on the same HS, there can only be one cause: they left.
                 if (invitingUserId.endsWith(`:${MatrixClientPeg.get().getDomain()}`)) {
@@ -602,6 +602,23 @@ export class RoomViewStore extends EventEmitter {
                     description = _t("The person who invited you has already left, or their server is offline.");
                 }
             }
+
+            // provide a more detailed error than "No known servers" when attempting to
+            // join using a room ID and no via servers
+            if (roomId === this.state.roomId && this.state.viaServers.length === 0) {
+                description = (
+                    <div>
+                        {_t(
+                            "You attempted to join using a room ID without providing a list " +
+                                "of servers to join through. Room IDs are internal identifiers and " +
+                                "cannot be used to join a room without additional information.",
+                        )}
+                        <br />
+                        <br />
+                        {_t("If you know a room address, try joining through that instead.")}
+                    </div>
+                );
+            }
         }
 
         Modal.createDialog(ErrorDialog, {
@@ -615,7 +632,9 @@ export class RoomViewStore extends EventEmitter {
             joining: false,
             joinError: payload.err,
         });
-        this.showJoinRoomError(payload.err, payload.roomId);
+        if (payload.err) {
+            this.showJoinRoomError(payload.err, payload.roomId);
+        }
     }
 
     public reset(): void {

test/stores/RoomViewStore-test.ts

@@ -14,7 +14,8 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { Room } from "matrix-js-sdk/src/matrix";
+import { mocked } from "jest-mock";
+import { MatrixError, Room } from "matrix-js-sdk/src/matrix";
 import { sleep } from "matrix-js-sdk/src/utils";
 
 import { RoomViewStore } from "../../src/stores/RoomViewStore";
@@ -284,6 +285,29 @@ describe("RoomViewStore", function () {
         await viewCall();
     });
 
+    it("should display an error message when the room is unreachable via the roomId", async () => {
+        // When
+        // View and wait for the room
+        dis.dispatch({ action: Action.ViewRoom, room_id: roomId });
+        await untilDispatch(Action.ActiveRoomChanged, dis);
+        // Generate error to display the expected error message
+        const error = new MatrixError(undefined, 404);
+        roomViewStore.showJoinRoomError(error, roomId);
+
+        // Check the modal props
+        expect(mocked(Modal).createDialog.mock.calls[0][1]).toMatchSnapshot();
+    });
+
+    it("should display the generic error message when the roomId doesnt match", async () => {
+        // When
+        // Generate error to display the expected error message
+        const error = new MatrixError({ error: "my 404 error" }, 404);
+        roomViewStore.showJoinRoomError(error, roomId);
+
+        // Check the modal props
+        expect(mocked(Modal).createDialog.mock.calls[0][1]).toMatchSnapshot();
+    });
+
     describe("when listening to a voice broadcast", () => {
         let voiceBroadcastPlayback: VoiceBroadcastPlayback;
 

test/stores/__snapshots__/RoomViewStore-test.ts.snap

@@ -1,5 +1,24 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`RoomViewStore should display an error message when the room is unreachable via the roomId 1`] = `
+{
+  "description": <div>
+    You attempted to join using a room ID without providing a list of servers to join through. Room IDs are internal identifiers and cannot be used to join a room without additional information.
+    <br />
+    <br />
+    If you know a room address, try joining through that instead.
+  </div>,
+  "title": "Failed to join",
+}
+`;
+
+exports[`RoomViewStore should display the generic error message when the roomId doesnt match 1`] = `
+{
+  "description": "MatrixError: [404] my 404 error",
+  "title": "Failed to join",
+}
+`;
+
 exports[`RoomViewStore when recording a voice broadcast and trying to view a call, it should not actually view it and show the info dialog 1`] = `
 [MockFunction] {
   "calls": [