It looks like the templates for Mojo enum types don't take into consideration variants. Adding an enum to test_wtf_types.mojom generates compile errors because of the absolute symbol name referring to ::mojo::test::internal instead of ::mojo::test::blink::internal.

--- a/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
+++ b/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom
@@ -4,7 +4,13 @@
 
 module mojo.test;
 
+enum MojoBoolean {
+  MOJO_TRUE,
+  MOJO_FALSE,
+};
+
 struct TestWTFCodeGeneration {
+  MojoBoolean test_bool;
   string str;
   string? nullable_str;
   array<string> strs;

In file included from ../../mojo/public/cpp/bindings/tests/wtf_types_unittest.cc:13:
gen/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom-blink.h:51:10: error: no member named 'internal' in namespace 'mojo::test'; did you mean simply 'internal'?
  return ::mojo::test::internal::MojoBoolean_Data::IsKnownValue(
         ^~~~~~~~~~~~~~~~~~~~~~
         internal
gen/mojo/public/interfaces/bindings/tests/test_wtf_types.mojom-blink-internal.h:27:11: note: 'internal' declared here
namespace internal {
          ^
1 error generated.