Add TemplateNullType

This fixes issues like #13048. Currently, T of (A|null) subtracted by A is T of mixed

Author: SplotyCode

src/Type/Generic/TemplateNullType.php

@@ -0,0 +1,38 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PHPStan\Type\Generic;
+
+use PHPStan\Type\NullType;
+use PHPStan\Type\Traits\UndecidedComparisonCompoundTypeTrait;
+use PHPStan\Type\Type;
+
+final class TemplateNullType extends NullType implements TemplateType
+{
+	/** @use TemplateTypeTrait<NullType> */
+	use TemplateTypeTrait;
+	use UndecidedComparisonCompoundTypeTrait;
+
+
+	/**
+	 * @param non-empty-string $name
+	 */
+	public function __construct(
+		TemplateTypeScope $scope,
+		TemplateTypeStrategy $templateTypeStrategy,
+		TemplateTypeVariance $templateTypeVariance,
+		string $name,
+		NullType $bound,
+		?Type $default,
+	)
+	{
+		parent::__construct();
+		$this->scope = $scope;
+		$this->strategy = $templateTypeStrategy;
+		$this->variance = $templateTypeVariance;
+		$this->name = $name;
+		$this->bound = $bound;
+		$this->default = $default;
+	}
+}

src/Type/Generic/TemplateTypeFactory.php

@@ -15,6 +15,7 @@
 use PHPStan\Type\IterableType;
 use PHPStan\Type\KeyOfType;
 use PHPStan\Type\MixedType;
+use PHPStan\Type\NullType;
 use PHPStan\Type\ObjectShapeType;
 use PHPStan\Type\ObjectType;
 use PHPStan\Type\ObjectWithoutClassType;
@@ -112,6 +113,10 @@ public static function create(TemplateTypeScope $scope, string $name, ?Type $bou
 			return new TemplateIterableType($scope, $strategy, $variance, $name, $bound, $default);
 		}
 
+		if ($bound instanceof NullType && ($boundClass === NullType::class || $bound instanceof TemplateType)) {
+			return new TemplateNullType($scope, $strategy, $variance, $name, $bound, $default);
+		}
+
 		return new TemplateMixedType($scope, $strategy, $variance, $name, new MixedType(true), $default);
 	}