GraphQL\Type\SchemaValidator::interfacePossibleTypesMustImplementTheInterfaceRule PHP Method

    public static function interfacePossibleTypesMustImplementTheInterfaceRule()
    {
        return function ($context) {
            /** @var Schema $schema */
            $schema = $context['schema'];
            $typeMap = $schema->getTypeMap();
            $errors = [];
            foreach ($typeMap as $typeName => $type) {
                if ($type instanceof InterfaceType) {
                    $possibleTypes = $type->getPossibleTypes();
                    foreach ($possibleTypes as $possibleType) {
                        if (!in_array($type, $possibleType->getInterfaces())) {
                            $errors[] = new Error("{$possibleType} is a possible type of interface {$type} but does " . "not implement it!");
                        }
                    }
                }
            }
            return !empty($errors) ? $errors : null;
        };
    }

 public function testRejectsWhenAPossibleTypeDoesNotImplementTheInterface()
 {
     // rejects when a possible type does not implement the interface
     $InterfaceType = new InterfaceType(['name' => 'InterfaceType', 'fields' => []]);
     $SubType = new ObjectType(['name' => 'SubType', 'fields' => [], 'interfaces' => []]);
     InterfaceType::addImplementationToInterfaces($SubType, [$InterfaceType]);
     // Sanity check.
     $this->assertEquals(1, count($InterfaceType->getPossibleTypes()));
     $this->assertEquals($SubType, $InterfaceType->getPossibleTypes()[0]);
     $schema = new Schema($InterfaceType);
     $validationResult = SchemaValidator::validate($schema, [SchemaValidator::interfacePossibleTypesMustImplementTheInterfaceRule()]);
     $this->assertSame(false, $validationResult->isValid);
     $this->assertSame(1, count($validationResult->errors));
     $this->assertSame('SubType is a possible type of interface InterfaceType but does not ' . 'implement it!', $validationResult->errors[0]->message);
 }