Allow Mojo structs as map keys

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

Index: mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_definition.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_definition.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_definition.tmpl
index e905904174d920db89fbd7ef087b7fb70b890929..d12554c19e07130eac5549dc5213f3c9716e1df3 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_definition.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/wrapper_union_class_definition.tmpl
@@ -65,3 +65,18 @@ void {{union.name}}::DestroyActive() {
 {%- endfor %}
   }
 }
+
+{%- if union|is_hashable %}
+size_t {{union.name}}::Hash(size_t seed) const {
+  switch (tag_) {

[Sam McNally, 2016/09/21 04:57:58]

Should |tag_| influence the hash?

[tibell, 2016/09/21 07:10:54]

Done.

We better if we have an empty union (e.g. an enum) or otherwise they will all
hash to the same value!

+{%  for field in union.fields %}
+    case Tag::{{field.name|upper}}:
+      return mojo::internal::Hash(seed, data_.{{field.name}});
+{%- endfor %}
+    default:
+      NOTREACHED();
+      return seed;
+  }
+}
+
+{%- endif %}