Make the fuchsia mojo/public repo the source of truth.

mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl

-{%- set class_name = union.name ~ "_Data" -%}
-{%- set enum_name = union.name ~ "_Tag" -%}
-{%- import "struct_macros.tmpl" as struct_macros %}
-
-class {{class_name}} {
- public:
-  // Used to identify Mojom Union Data Classes.
-  typedef void MojomUnionDataType;
-  static {{class_name}}* New(mojo::internal::Buffer* buf);
-  {{class_name}}();
-  // Do nothing in the destructor since it won't be called.
-  ~{{class_name}}() {}
-
-  static mojo::internal::ValidationError Validate(
-      const void* data,
-      mojo::internal::BoundsChecker* bounds_checker,
-      bool inlined,
-      std::string* err);
-
-  bool is_null() const {
-    return size == 0;
-  }
-
-  void set_null();
-
-  enum class {{enum_name}} : uint32_t {
-{%  for field in union.fields %}
-    {{field.name|upper}},
-{%- endfor %}
-    __UNKNOWN__ = 0xFFFFFFFF,
-  };
-
-  // A note on layout:
-  // "Each non-static data member is allocated as if it were the sole member of
-  // a struct." - Section 9.5.2 ISO/IEC 14882:2011 (The C++ Spec)
-  union MOJO_ALIGNAS(8) Union_ {
-{%- for field in union.fields %}
-{%-   if field.kind.spec == 'b' %}
-    uint8_t f_{{field.name}} : 1;
-{%-   else %}
-    {{field.kind|cpp_union_field_type}} f_{{field.name}};
-{%-   endif %}
-{%- endfor %}
-    uint64_t unknown;
-  };
-
-  uint32_t size;
-  {{enum_name}} tag;
-  Union_ data;
-
-  void EncodePointersAndHandles(std::vector<mojo::Handle>* handles);
-  void DecodePointersAndHandles(std::vector<mojo::Handle>* handles);
-};
-static_assert(sizeof({{class_name}}) == 16,
-              "Bad sizeof({{class_name}})");
-static_assert(sizeof({{class_name}}::Union_) == 8,
-              "Bad sizeof({{class_name}}::Union_)");