docs: private void checkUnionTypeExtensions(List<GraphQLError> errors, TypeDefinitionRegistry typeRegistry) {
          typeRegistry.unionTypeExtensions()
                  .forEach((name, extensions) -> {
                      checkTypeExtensionHasCorrespondingType(errors, typeRegistry, name, extensions, UnionTypeDefinition.class);
                      checkTypeExtensionDirectiveRedefinition(errors, typeRegistry, name, extensions, UnionTypeDefinition.class);
  
                      extensions.forEach(extension -> {
                          List<TypeName> memberTypes = extension.getMemberTypes().stream()
                                  .map(t -> TypeInfo.typeInfo(t).getTypeName()).collect(Collectors.toList());
  
                          checkNamedUniqueness(errors, memberTypes, TypeName::getName,
                                  (namedMember, memberType) -> new NonUniqueNameError(extension, namedMember));
  
                          memberTypes.forEach(
                                  memberType -> {
                                      Optional<ObjectTypeDefinition> unionTypeDefinition = typeRegistry.getType(memberType, ObjectTypeDefinition.class);
                                      if (!unionTypeDefinition.isPresent()) {
                                          errors.add(new MissingTypeError("union member", extension, memberType));
                                      }
                                  }
                          );
                      });
                  });
      }