Allow DynamicStruct::Reader/Builder.as<AnyStruct>

Also fix AnyStruct::Reader/Builder as methods

c++/src/capnp/any-test.c++

@@ -114,6 +114,12 @@ TEST(Any, AnyStruct) {
   checkTestMessage(root.getAnyPointerField().getAs<TestAllTypes>());
   checkTestMessage(root.asReader().getAnyPointerField().getAs<TestAllTypes>());
 
+  auto all_types = root.getAnyPointerField().getAs<AnyStruct>().as<TestAllTypes>();
+  auto all_types_reader = root.getAnyPointerField().getAs<AnyStruct>().asReader().as<TestAllTypes>();
+  all_types.setInt32Field(100);
+  EXPECT_EQ(100, all_types.getInt32Field());
+  EXPECT_EQ(100, all_types_reader.getInt32Field());
+
   EXPECT_EQ(48, root.getAnyPointerField().getAs<AnyStruct>().getDataSection().size());
   EXPECT_EQ(20, root.getAnyPointerField().getAs<AnyStruct>().getPointerSection().size());
 

c++/src/capnp/any.h

@@ -436,7 +436,9 @@ public:
   }
 
   template <typename T>
-  ReaderFor<T> as();
+  ReaderFor<T> as() const {
+    return typename T::Reader(_reader);
+  }
   // T must be a struct type.
 private:
   _::StructReader _reader;
@@ -465,6 +467,12 @@ public:
 
   inline operator Reader() const { return Reader(_builder.asReader()); }
   inline Reader asReader() const { return Reader(_builder.asReader()); }
+
+  template <typename T>
+  BuilderFor<T> as() {
+    return typename T::Builder(_builder);
+  }
+  // T must be a struct type.
 private:
   _::StructBuilder _builder;
   friend class Orphanage;

c++/src/capnp/dynamic-test.c++

@@ -222,6 +222,15 @@ TEST(DynamicApi, DynamicAnyPointers) {
   }
 }
 
+TEST(DynamicApi, DynamicAnyStructs) {
+  MallocMessageBuilder builder;
+  auto root = builder.initRoot<DynamicStruct>(Schema::from<TestAllTypes>());
+
+  root.as<AnyStruct>().as<TestAllTypes>().setInt8Field(123);
+  EXPECT_EQ(root.get("int8Field").as<int8_t>(), 123);
+  EXPECT_EQ(root.asReader().as<AnyStruct>().as<TestAllTypes>().getInt8Field(), 123);
+}
+
 #define EXPECT_MAYBE_EQ(name, exp, expected, actual) \
   KJ_IF_MAYBE(name, exp) { \
     EXPECT_EQ(expected, actual); \

c++/src/capnp/dynamic.h

@@ -1435,6 +1435,7 @@ typename T::Reader DynamicStruct::Reader::as() const {
   schema.requireUsableAs<T>();
   return typename T::Reader(reader);
 }
+
 template <typename T>
 typename T::Builder DynamicStruct::Builder::as() {
   static_assert(kind<T>() == Kind::STRUCT,
@@ -1456,6 +1457,16 @@ inline DynamicStruct::Reader DynamicStruct::Builder::asReader() const {
   return DynamicStruct::Reader(schema, builder.asReader());
 }
 
+template <>
+inline AnyStruct::Reader DynamicStruct::Reader::as<AnyStruct>() const {
+  return AnyStruct::Reader(reader);
+}
+
+template <>
+inline AnyStruct::Builder DynamicStruct::Builder::as<AnyStruct>() {
+  return AnyStruct::Builder(builder);
+}
+
 template <typename T>
 typename T::Pipeline DynamicStruct::Pipeline::releaseAs() {
   static_assert(kind<T>() == Kind::STRUCT,