It's okay to have always-throwing expression in arrow function

Author: ondrejmirtes

src/Php/PhpVersion.php

@@ -272,4 +272,9 @@ public function supportsReadOnlyAnonymousClasses(): bool
 		return $this->versionId >= 80300;
 	}
 
+	public function supportsNeverReturnTypeInArrowFunction(): bool
+	{
+		return $this->versionId >= 80200;
+	}
+
 }

src/Rules/Functions/ArrowFunctionReturnTypeRule.php

@@ -9,6 +9,7 @@
 use PHPStan\Rules\FunctionReturnTypeCheck;
 use PHPStan\Rules\Rule;
 use PHPStan\ShouldNotHappenException;
+use PHPStan\Type\NeverType;
 use PHPStan\Type\ObjectType;
 use PHPStan\Type\Type;
 
@@ -43,6 +44,16 @@ public function processNode(Node $node, Scope $scope): array
 			return [];
 		}
 
+		$exprType = $scope->getType($originalNode->expr);
+		if (
+			$returnType instanceof NeverType
+			&& $returnType->isExplicit()
+			&& $exprType instanceof NeverType
+			&& $exprType->isExplicit()
+		) {
+			return [];
+		}
+
 		return $this->returnTypeCheck->checkReturnType(
 			$scope,
 			$returnType,

src/Rules/Functions/ExistingClassesInArrowFunctionTypehintsRule.php

@@ -4,16 +4,21 @@
 
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
+use PHPStan\Php\PhpVersion;
 use PHPStan\Rules\FunctionDefinitionCheck;
 use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+use PHPStan\Type\NonAcceptingNeverType;
+use PHPStan\Type\ParserNodeTypeToPHPStanType;
+use function array_merge;
 
 /**
  * @implements Rule<Node\Expr\ArrowFunction>
  */
 class ExistingClassesInArrowFunctionTypehintsRule implements Rule
 {
 
-	public function __construct(private FunctionDefinitionCheck $check)
+	public function __construct(private FunctionDefinitionCheck $check, private PhpVersion $phpVersion)
 	{
 	}
 
@@ -24,7 +29,17 @@ public function getNodeType(): string
 
 	public function processNode(Node $node, Scope $scope): array
 	{
-		return $this->check->checkAnonymousFunction(
+		$messages = [];
+		if ($node->returnType !== null && !$this->phpVersion->supportsNeverReturnTypeInArrowFunction()) {
+			$returnType = ParserNodeTypeToPHPStanType::resolve($node->returnType, $scope->isInClass() ? $scope->getClassReflection() : null);
+			if ($returnType instanceof NonAcceptingNeverType) {
+				$messages[] = RuleErrorBuilder::message('Never return type in arrow function is supported only on PHP 8.2 and later.')
+					->nonIgnorable()
+					->build();
+			}
+		}
+
+		return array_merge($messages, $this->check->checkAnonymousFunction(
 			$scope,
 			$node->getParams(),
 			$node->getReturnType(),
@@ -33,7 +48,7 @@ public function processNode(Node $node, Scope $scope): array
 			'Anonymous function uses native union types but they\'re supported only on PHP 8.0 and later.',
 			'Parameter $%s of anonymous function has unresolvable native type.',
 			'Anonymous function has unresolvable native return type.',
-		);
+		));
 	}
 
 }

tests/PHPStan/Rules/Functions/ArrowFunctionReturnTypeRuleTest.php

@@ -39,6 +39,10 @@ public function testRule(): void
 				'Anonymous function should return int but returns string.',
 				14,
 			],
+			[
+				'Anonymous function should never return but return statement found.',
+				44,
+			],
 		]);
 	}
 

tests/PHPStan/Rules/Functions/ExistingClassesInArrowFunctionTypehintsRuleTest.php

@@ -21,7 +21,7 @@ class ExistingClassesInArrowFunctionTypehintsRuleTest extends RuleTestCase
 	protected function getRule(): Rule
 	{
 		$broker = $this->createReflectionProvider();
-		return new ExistingClassesInArrowFunctionTypehintsRule(new FunctionDefinitionCheck($broker, new ClassCaseSensitivityCheck($broker, true), new UnresolvableTypeHelper(), new PhpVersion($this->phpVersionId), true, false));
+		return new ExistingClassesInArrowFunctionTypehintsRule(new FunctionDefinitionCheck($broker, new ClassCaseSensitivityCheck($broker, true), new UnresolvableTypeHelper(), new PhpVersion($this->phpVersionId), true, false), new PhpVersion(PHP_VERSION_ID));
 	}
 
 	public function testRule(): void
@@ -147,4 +147,18 @@ public function testIntersectionTypes(int $phpVersion, array $errors): void
 		$this->analyse([__DIR__ . '/data/arrow-function-intersection-types.php'], $errors);
 	}
 
+	public function testNever(): void
+	{
+		$errors = [];
+		if (PHP_VERSION_ID < 80200) {
+			$errors = [
+				[
+					'Never return type in arrow function is supported only on PHP 8.2 and later.',
+					6,
+				],
+			];
+		}
+		$this->analyse([__DIR__ . '/data/arrow-function-never.php'], $errors);
+	}
+
 }

tests/PHPStan/Rules/Functions/data/arrow-function-never.php

@@ -0,0 +1,7 @@
+<?php // lint >= 7.4
+
+namespace ArrowFunctionNever;
+
+function (): void {
+	$g = fn (): never => throw new \Exception();
+};

tests/PHPStan/Rules/Functions/data/arrow-functions-return-type.php

@@ -33,3 +33,15 @@ public function doBar(): void
 }
 
 static fn (int $value): iterable => yield $value;
+
+class Baz
+{
+
+	public function doFoo(): void
+	{
+		$f = fn () => throw new \Exception();
+		$g = fn (): never => throw new \Exception();
+		$g = fn (): never => 1;
+	}
+
+}