Fix/nullable templates

phpstan-baseline.neon

@@ -1251,6 +1251,12 @@ parameters:
 			count: 2
 			path: src/Type/Generic/TemplateMixedType.php
 
+		-
+			message: '#^Doing instanceof PHPStan\\Type\\IntersectionType is error\-prone and deprecated\.$#'
+			identifier: phpstanApi.instanceofType
+			count: 3
+			path: src/Type/Generic/TemplateNullType.php
+
 		-
 			message: '#^Doing instanceof PHPStan\\Type\\IntersectionType is error\-prone and deprecated\.$#'
 			identifier: phpstanApi.instanceofType
@@ -1335,6 +1341,12 @@ parameters:
 			count: 1
 			path: src/Type/Generic/TemplateTypeFactory.php
 
+		-
+			message: '#^Doing instanceof PHPStan\\Type\\NullType is error\-prone and deprecated\. Use Type\:\:isNull\(\) instead\.$#'
+			identifier: phpstanApi.instanceofType
+			count: 1
+			path: src/Type/Generic/TemplateTypeFactory.php
+
 		-
 			message: '#^Doing instanceof PHPStan\\Type\\ObjectShapeType is error\-prone and deprecated\. Use Type\:\:isObject\(\) and Type\:\:hasProperty\(\) instead\.$#'
 			identifier: phpstanApi.instanceofType

src/Type/Generic/TemplateNullType.php

@@ -0,0 +1,37 @@
+<?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);
 	}
 

tests/PHPStan/Rules/Comparison/MatchExpressionRuleTest.php

@@ -435,4 +435,11 @@ public function testPropertyHooks(): void
 		]);
 	}
 
+
+	#[RequiresPhp('>= 8.1')]
+	public function testBug13048(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13048.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Comparison/data/bug-13048.php

@@ -0,0 +1,21 @@
+<?php // lint >= 8.1
+
+namespace Bug13048;
+
+enum IndexBy {
+	case A;
+	case B;
+}
+
+/**
+ * @template T of IndexBy|null
+ * @param T $indexBy
+ */
+function run(?IndexBy $indexBy = null): ?string
+{
+	return match ($indexBy) {
+		IndexBy::A => 'by A',
+		IndexBy::B => 'by B',
+		null => null,
+	};
+}