fix(generator): validate alias capture member names

CHANGELOG.md

@@ -8,6 +8,8 @@ See [Conventional Commits](https://conventionalcommits.org) for commit guideline
 
 ### Fixed
 
+* Reject generator aliases whose final passthrough policy makes an inherited
+  field collide with the generated additional-properties member.
 * Preserve round trips for discriminated map codecs with strict output schemas,
   including values used as another codec's output, without narrowing permissive
   intermediate output policies.

packages/ack_generator/CHANGELOG.md

@@ -7,6 +7,8 @@
 
 ### Fixed
 
+* Validate inherited field names against an alias's final capture policy before
+  generation, rejecting collisions with the additional-properties member.
 * Reject unproven object-shape changes inside preserved boundaries, including
   aliases and fluent codecs, rather than copying opaque values as JSON.
 * Preserve native codec boundary types when inferring `Ack.preserveBoundary`

packages/ack_generator/lib/src/analyzer/schema_model_graph_builder.dart

@@ -388,16 +388,6 @@ final class SchemaModelGraphBuilder extends ModelGraphBuilderBase {
           element: declaration.element,
         );
       }
-      final problem = ackSchemaFirstFieldNameProblem(
-        jsonKey,
-        capturesExtras: additionalProperties,
-      );
-      if (problem != null) {
-        throw InvalidGenerationSource(
-          '$path.$jsonKey $problem',
-          element: declaration.element,
-        );
-      }
       final fieldPath = '$path.$jsonKey';
       final fieldChain = parseSchemaChain(entry.value);
       final fieldMetadata = await _schemaMetadata(
@@ -1239,6 +1229,16 @@ final class SchemaModelGraphBuilder extends ModelGraphBuilderBase {
       }
       if (node is AckObjectModelNode) {
         for (final field in node.fields) {
+          final problem = ackSchemaFirstFieldNameProblem(
+            field.dartName,
+            capturesExtras: node.additionalProperties,
+          );
+          if (problem != null) {
+            throw InvalidGenerationSource(
+              '${node.id.declarationName}.${field.dartName} $problem',
+              element: declaration.element,
+            );
+          }
           checkMember(field.dartName);
         }
         if (node.discriminatorKey case final discriminator?) {

packages/ack_generator/test/integration/generated_names_contract_test.dart

@@ -68,6 +68,61 @@ final aliasSchema = originalSchema;
     );
   });
 
+  for (final alias in [
+    'final openSchema = baseSchema.passthrough();',
+    'final openSchema = baseSchema.copyWith(additionalProperties: true);',
+    'ObjectSchema get openSchema => baseSchema.passthrough();',
+    '''final middleSchema = baseSchema;
+@Schemable()
+final openSchema = middleSchema.passthrough();''',
+  ]) {
+    test('rejects inherited capture-member collisions: $alias', () async {
+      await _expectFailure(
+        '''
+$_head
+@Schemable()
+final baseSchema = Ack.object({'additionalProperties': Ack.string()});
+@Schemable()
+$alias
+''',
+        ['openSchema.additionalProperties', 'additional-properties member'],
+      );
+    });
+  }
+
+  for (final overrides in [
+    '.passthrough().strict()',
+    '.copyWith(additionalProperties: true).copyWith(additionalProperties: false)',
+  ]) {
+    test('permits inherited additionalProperties after $overrides', () async {
+      final readerWriter = TestReaderWriter(rootPackage: 'test_pkg');
+      await readerWriter.testing.loadIsolateSources();
+      await testBuilder(
+        ackModelBuilder(BuilderOptions.empty),
+        {
+          'test_pkg|lib/models.dart':
+              '''
+$_head
+@Schemable()
+final baseSchema = Ack.object({'additionalProperties': Ack.string()});
+@Schemable()
+final aliasSchema = baseSchema$overrides;
+''',
+        },
+        readerWriter: readerWriter,
+        outputs: {
+          'test_pkg|lib/models.g.dart': decodedMatches(
+            allOf(
+              contains('class Alias'),
+              contains('final String additionalProperties;'),
+              isNot(contains('Map<String, Object?> additionalProperties')),
+            ),
+          ),
+        },
+      );
+    });
+  }
+
   for (final discriminator in ['Pet', 'Cat']) {
     test('rejects discriminator matching class $discriminator', () async {
       await _expectFailure(