message: [C++] Fix union copy constructor to work with nested structs
diff: @@ -1534,7 +1534,8 @@ class CppGenerator : public BaseGenerator {
  code_.SetValue("TYPE", GetUnionElement(ev, true, opts_));
  code_ += " case {{LABEL}}: {";
  bool copyable = true;
  - if (ev.union_type.base_type == BASE_TYPE_STRUCT) {
  + if (ev.union_type.base_type == BASE_TYPE_STRUCT &&
  + !ev.union_type.struct_def->fixed) {
  // Don't generate code to copy if table is not copyable.
  // TODO(wvo): make tables copyable instead.
  for (auto fit = ev.union_type.struct_def->fields.vec.begin();
  