Fix JSON field handlers not triggering on union members of annotated types.

c++/src/capnp/compat/json-test.c++

@@ -975,6 +975,21 @@ KJ_TEST("rename fields") {
   }
 }
 
+KJ_TEST("base64 union encoded correctly") {
+  // At one point field handlers were not correctly applied when the field was a member of a union
+  // in a type that was handled by annotation.
+
+  JsonCodec json;
+  json.handleByAnnotation<TestBase64Union>();
+  json.setPrettyPrint(true);
+
+  MallocMessageBuilder message;
+  auto root = message.getRoot<TestBase64Union>();
+  root.initFoo(5);
+
+  KJ_EXPECT(json.encode(root) == "{\"foo\": \"AAAAAAA=\"}", json.encode(root));
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp

c++/src/capnp/compat/json-test.capnp

@@ -114,3 +114,10 @@ enum TestJsonAnnotatedEnum {
   baz @2 $Json.name("renamed-baz");
   qux @3;
 }
+
+struct TestBase64Union {
+  union {
+    foo @0 :Data $Json.base64;
+    bar @1 :Text;
+  }
+}

c++/src/capnp/compat/json.c++

@@ -1255,7 +1255,7 @@ private:
           // When we have an explicit union discriminant, we don't need to encode void fields.
         } else {
           flattenedFields.add(FlattenedField {
-              prefix, info.name, which->getType(), reader.get(*which) });
+              prefix, info.name, *which, reader.get(*which) });
         }
       }
     }