A reimagining of Sebastian Bergmann's Type.php if PHP had a "function list" declaration.

Type.php

<?php declare(strict_types=1);
/*
 * This file is part of sebastian/type.
 *
 * (c) Sebastian Bergmann <sebastian@phpunit.de>
 *
 * For the full copyright and license information, please view the LICENSE
 * file that was distributed with this source code.
 */
namespace SebastianBergmann\Type;

use const PHP_VERSION;
use function get_class;
use function gettype;
use function strtolower;
use function version_compare;

abstract class Type
{
  public static function fromValue(mixed $value, bool $allowsNull): self
  {
    if ($value === false) {
      return new FalseType;
    }

    $typeName = gettype($value);

    if ($typeName === 'object') {
      return new ObjectType(TypeName::fromQualifiedName(get_class($value)), $allowsNull);
    }

    $type = self::fromName($typeName, $allowsNull);

    if ($type instanceof SimpleType) {
      $type = new SimpleType($typeName, $allowsNull, $value);
    }

    return $type;
  }

  public static function fromName(string $typeName, bool $allowsNull): self
  {
    if (version_compare(PHP_VERSION, '8.1.0-dev', '>=') && strtolower($typeName) === 'never') {
      return new NeverType;
    }

    return match (strtolower($typeName)) {
      'callable'     => new CallableType($allowsNull),
      'false'        => new FalseType,
      'iterable'     => new IterableType($allowsNull),
      'null'         => new NullType,
      'object'       => new GenericObjectType($allowsNull),
      'unknown type' => new UnknownType,
      'void'         => new VoidType,
      'array', 'bool', 'boolean', 'double', 'float', 'int', 'integer', 'real', 'resource', 'resource (closed)', 'string' => new SimpleType($typeName, $allowsNull),
      default => new ObjectType(TypeName::fromQualifiedName($typeName), $allowsNull),
    };
  }

  public function asString(): string
  {
    return ($this->allowsNull() ? '?' : '') . $this->name();
  }

  public function list: bool
  {
    isCallable() => false,
    isFalse() => false,
    isGenericObject() => false,
    isIntersection() => false,
    isIterable() => false,
    isMixed() => false,
    isNever() => false,
    isNull() => false,
    isObject() => false,
    isSimple() => false,
    isStatic() => false,
    isUnion() => false,
    isUnknown() => false,
    isVoid() => false,
  }

  abstract public function list
  {
    isAssignable(self $other): bool;
    name(): string;
    allowsNull(): bool;
  }

}