Fix bug where DynamicValue::Builder was not assignable.

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

@@ -439,6 +439,24 @@ TEST(DynamicApi, SetDataFromText) {
   EXPECT_EQ(data("foo"), root.get("dataField").as<Data>());
 }
 
+TEST(DynamicApi, BuilderAssign) {
+  MallocMessageBuilder builder;
+  auto root = builder.initRoot<DynamicStruct>(Schema::from<TestAllTypes>());
+
+  // Declare upfront, assign later.
+  DynamicValue::Builder value;
+  DynamicStruct::Builder structValue = nullptr;
+  DynamicList::Builder listValue = nullptr;
+
+  value = root.get("structField");
+  structValue = value.as<DynamicStruct>();
+  structValue.set("int32Field", 123);
+
+  value = root.init("int32List", 1);
+  listValue = value.as<DynamicList>();
+  listValue.set(0, 123);
+}
+
 }  // namespace
 }  // namespace _ (private)
 }  // namespace capnp

c++/src/capnp/dynamic.h

@@ -613,6 +613,8 @@ public:
 
   inline Builder(Builder& other) { memcpy(this, &other, sizeof(*this)); }
   inline Builder(Builder&& other) { memcpy(this, &other, sizeof(*this)); }
+  inline Builder& operator=(Builder& other) { memcpy(this, &other, sizeof(*this)); }
+  inline Builder& operator=(Builder&& other) { memcpy(this, &other, sizeof(*this)); }
   static_assert(__has_trivial_copy(StructSchema) && __has_trivial_copy(ListSchema),
                 "Assumptions made here do not hold.");
   // Hack:  We know this type is trivially constructable but the use of DisallowConstCopy causes