Throw if the main argument in plurals does not match the argument list.

pkgs/intl_translation/CHANGELOG.md

@@ -1,6 +1,7 @@
 ## 0.21.0-wip
   * BREAKING CHANGE: Update `dart_style` to `^3.0.0`
   * Allow analyzer `>=6.3.0 <8.0.0`
+  * Throw if the main argument in plurals and selects does not match the argument list.
 
 ## 0.20.1
   * Add topics to `pubspec.yaml`

pkgs/intl_translation/lib/src/messages/submessages/submessage.dart

@@ -95,7 +95,14 @@ abstract class SubMessage extends ComplexMessage {
   List toJson() {
     var json = [];
     json.add(dartMessageName);
-    json.add(arguments.indexOf(mainArgument));
+    var indexOf = arguments.indexOf(mainArgument);
+    if (indexOf == -1) {
+      // This is an error as this should have been checked in
+      // lib/visitors/plural_gender_visitor.dart already.
+      throw ArgumentError('The argument `$mainArgument` could not be found in '
+          '`$arguments`.');
+    }
+    json.add(indexOf);
     for (var arg in codeAttributeNames) {
       json.add(this[arg]?.toJson());
     }

pkgs/intl_translation/lib/visitors/plural_gender_visitor.dart

@@ -137,6 +137,13 @@ class PluralAndGenderVisitor extends SimpleAstVisitor<void> {
       extraction.warnings.add(errString);
     }
 
+    if (!message.arguments.contains(message.mainArgument)) {
+      throw Exception(
+          'Argument `${message.mainArgument}` could not be found in '
+          '${message.arguments} while processing $node. The parent argument '
+          'name must match the one in the `Intl.` call.');
+    }
+
     return message;
   }
 }