Fix phpstan/phpstan#13806: String cast should create throw points for __toString()

- Added throw point creation in CastStringHandler when __toString() method exists
- Respects explicit throw types from __toString() and implicit throws configuration
- New regression test in tests/PHPStan/Rules/Exceptions/data/bug-13806.php
- The root cause was that CastStringHandler tracked impurity but not throw points for __toString()

Author: staabm

src/Analyser/ExprHandler/CastStringHandler.php

@@ -10,11 +10,16 @@
 use PHPStan\Analyser\ExpressionResultStorage;
 use PHPStan\Analyser\ExprHandler;
 use PHPStan\Analyser\ImpurePoint;
+use PHPStan\Analyser\InternalThrowPoint;
 use PHPStan\Analyser\MutatingScope;
 use PHPStan\Analyser\NodeScopeResolver;
+use PHPStan\DependencyInjection\AutowiredParameter;
 use PHPStan\DependencyInjection\AutowiredService;
 use PHPStan\Reflection\InitializerExprTypeResolver;
+use PHPStan\Type\NeverType;
+use PHPStan\Type\ObjectType;
 use PHPStan\Type\Type;
+use Throwable;
 use function sprintf;
 
 /**
@@ -26,6 +31,8 @@ final class CastStringHandler implements ExprHandler
 
 	public function __construct(
 		private InitializerExprTypeResolver $initializerExprTypeResolver,
+		#[AutowiredParameter(ref: '%exceptions.implicitThrows%')]
+		private bool $implicitThrows,
 	)
 	{
 	}
@@ -39,6 +46,7 @@ public function processExpr(NodeScopeResolver $nodeScopeResolver, Stmt $stmt, Ex
 	{
 		$exprResult = $nodeScopeResolver->processExprNode($stmt, $expr->expr, $scope, $storage, $nodeCallback, $context->enterDeep());
 		$impurePoints = $exprResult->getImpurePoints();
+		$throwPoints = $exprResult->getThrowPoints();
 
 		$exprType = $scope->getType($expr->expr);
 		$toStringMethod = $scope->getMethodReflection($exprType, '__toString');
@@ -52,6 +60,22 @@ public function processExpr(NodeScopeResolver $nodeScopeResolver, Stmt $stmt, Ex
 					$toStringMethod->isPure()->no(),
 				);
 			}
+
+			$throwType = $toStringMethod->getThrowType();
+			if ($throwType === null) {
+				$returnType = $toStringMethod->getVariants()[0]->getReturnType();
+				if ($returnType instanceof NeverType && $returnType->isExplicit()) {
+					$throwType = new ObjectType(Throwable::class);
+				}
+			}
+
+			if ($throwType !== null) {
+				if (!$throwType->isVoid()->yes()) {
+					$throwPoints[] = InternalThrowPoint::createExplicit($scope, $throwType, $expr, true);
+				}
+			} elseif ($this->implicitThrows) {
+				$throwPoints[] = InternalThrowPoint::createImplicit($scope, $expr);
+			}
 		}
 
 		$scope = $exprResult->getScope();
@@ -60,7 +84,7 @@ public function processExpr(NodeScopeResolver $nodeScopeResolver, Stmt $stmt, Ex
 			$scope,
 			hasYield: $exprResult->hasYield(),
 			isAlwaysTerminating: $exprResult->isAlwaysTerminating(),
-			throwPoints: $exprResult->getThrowPoints(),
+			throwPoints: $throwPoints,
 			impurePoints: $impurePoints,
 			truthyScopeCallback: static fn (): MutatingScope => $scope->filterByTruthyValue($expr),
 			falseyScopeCallback: static fn (): MutatingScope => $scope->filterByFalseyValue($expr),

tests/PHPStan/Rules/Exceptions/CatchWithUnthrownExceptionRuleTest.php

@@ -734,4 +734,9 @@ public function testBug9146NonStrict(): void
 		]);
 	}
 
+	public function testBug13806(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-13806.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Exceptions/data/bug-13806.php

@@ -0,0 +1,22 @@
+<?php // lint >= 8.0
+
+namespace Bug13806;
+
+function castToString(\Stringable|string $variable): string {
+	try {
+		$value = (string) $variable;
+	} catch(\Throwable) {
+		var_dump("Error thrown during string-conversion!");
+		$value = '';
+	}
+
+	return $value;
+}
+
+class MyString {
+	public function __toString(): never {
+		throw new \Exception();
+	}
+}
+
+castToString(new MyString());