fix(mf2): Catch errors thrown during MatchSelectorKeys (unicode-org/message-format-wg#828)

Author: XM5jDcsHTyGJtQqlCi

packages/mf2-messageformat/src/errors.ts

@@ -109,7 +109,12 @@ export class MessageResolutionError extends MessageError {
  */
 export class MessageSelectionError extends MessageError {
   declare type: 'bad-selector' | 'no-match';
-  constructor(type: typeof MessageSelectionError.prototype.type) {
+  cause?: unknown;
+  constructor(
+    type: typeof MessageSelectionError.prototype.type,
+    cause?: unknown
+  ) {
     super(type, `Selection error: ${type}`);
+    if (cause !== undefined) this.cause = cause;
   }
 }

packages/mf2-messageformat/src/select-pattern.ts

@@ -36,7 +36,13 @@ export function selectPattern(context: Context, message: Message): Pattern {
             if (key.type !== '*') sc.keys.add(key.value);
           }
         }
-        sc.best = sc.keys.size ? sc.selectKey(sc.keys) : null;
+        try {
+          sc.best = sc.keys.size ? sc.selectKey(sc.keys) : null;
+        } catch (error) {
+          context.onError(new MessageSelectionError('bad-selector', error));
+          sc.selectKey = () => null;
+          sc.best = null;
+        }
 
         // Leave out all candidate variants that aren't the best,
         // or only the catchall ones, if nothing else matches.