Fix regression in JSON annotations when flattening a non-group struct.

(Also make sure that external struct-unions are tested.)

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

@@ -844,10 +844,12 @@ R"({ "names-can_contain!anything Really": "foo",
   "testBase64": "ZnJlZA==",
   "testHex": "706c756768",
   "bUnion": "renamed-bar",
-  "bValue": 678 })"_kj;
+  "bValue": 678,
+  "externalUnion": {"type": "bar", "value": "cba"} })"_kj;
 
 static constexpr kj::StringPtr GOLDEN_ANNOTATED_REVERSE =
 R"({
+  "externalUnion": {"type": "bar", "value": "cba"},
   "bValue": 678,
   "bUnion": "renamed-bar",
   "testHex": "706c756768",
@@ -940,6 +942,8 @@ KJ_TEST("rename fields") {
 
     root.getBUnion().setBar(678);
 
+    root.initExternalUnion().initBar().setValue("cba");
+
     auto encoded = json.encode(root.asReader());
     KJ_EXPECT(encoded == GOLDEN_ANNOTATED, encoded);
 

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

@@ -82,6 +82,8 @@ struct TestJsonAnnotations {
     foo @20 :Text;
     bar @21 :UInt32 $Json.name("renamed-bar");
   }
+
+  externalUnion @22 :TestJsonAnnotations3;
 }
 
 struct TestJsonAnnotations2 {
@@ -89,6 +91,17 @@ struct TestJsonAnnotations2 {
   cycle @1 :TestJsonAnnotations;
 }
 
+struct TestJsonAnnotations3 $Json.discriminator(name = "type") {
+  union {
+    foo @0 :UInt32;
+    bar @1 :TestFlattenedStruct $Json.flatten();
+  }
+}
+
+struct TestFlattenedStruct {
+  value @0 :Text;
+}
+
 enum TestJsonAnnotatedEnum {
   foo @0;
   bar @1 $Json.name("renamed-bar");

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

@@ -1014,6 +1014,11 @@ public:
         if (flattened) {
           info.flattenHandler = subHandler;
         }
+      } else if (type.isStruct()) {
+        if (flattened) {
+          info.flattenHandler = codec.loadAnnotatedHandler(
+              type.asStruct(), nullptr, nullptr, dependencies);
+        }
       }
 
       bool isUnionMember = fieldProto.getDiscriminantValue() != schema::Field::NO_DISCRIMINANT;

c++/src/capnp/testdata/annotated.json

@@ -17,4 +17,5 @@
   "testBase64": "ZnJlZA==",
   "testHex": "706c756768",
   "bUnion": "renamed-bar",
-  "bValue": 678 }
+  "bValue": 678,
+  "externalUnion": {"type": "bar", "value": "cba"} }