code: private void checkEnumTypeExtensions(List<GraphQLError> errors, TypeDefinitionRegistry typeRegistry) {
          typeRegistry.enumTypeExtensions()
                  .forEach((name, extensions) -> {
                      checkTypeExtensionHasCorrespondingType(errors, typeRegistry, name, extensions, EnumTypeDefinition.class);
                      checkTypeExtensionDirectiveRedefinition(errors, typeRegistry, name, extensions, EnumTypeDefinition.class);
  
                      extensions.forEach(extension -> {
                          // field unique ness
                          List<EnumValueDefinition> enumValueDefinitions = extension.getEnumValueDefinitions();
                          checkNamedUniqueness(errors, enumValueDefinitions, EnumValueDefinition::getName,
                                  (namedField, enumValue) -> new NonUniqueNameError(extension, enumValue));
  
                          //
                          // enum values must be unique within a type extension
                          forEachBut(extension, extensions,
                                  otherTypeExt -> checkForEnumValueRedefinition(errors, otherTypeExt, otherTypeExt.getEnumValueDefinitions(), enumValueDefinitions));
  
                          //
                          // then check for field re-defs from the base type
                          Optional<EnumTypeDefinition> baseTypeOpt = typeRegistry.getType(extension.getName(), EnumTypeDefinition.class);
                          baseTypeOpt.ifPresent(baseTypeDef -> checkForEnumValueRedefinition(errors, extension, enumValueDefinitions, baseTypeDef.getEnumValueDefinitions()));
  
                      });
  
  
                  });
      }
docs: /*
  Enum type extensions have the potential to be invalid if incorrectly defined.
  
  The named type must already be defined and must be an Enum type.
  All values of an Enum type extension must be unique.
  All values of an Enum type extension must not already be a value of the original Enum.
  Any directives provided must not already apply to the original Enum type.