Fix false positive dead catch on property assignment

src/Analyser/NodeScopeResolver.php

@@ -139,6 +139,7 @@
 use PHPStan\Type\VoidType;
 use Throwable;
 use Traversable;
+use TypeError;
 use function array_fill_keys;
 use function array_filter;
 use function array_key_exists;
@@ -3314,11 +3315,24 @@ private function processAssignVar(
 				if ($propertyReflection->canChangeTypeAfterAssignment()) {
 					$scope = $scope->assignExpression($var, $assignedExprType);
 				}
+				if (!$propertyReflection->getWritableType()->isSuperTypeOf($assignedExprType)->yes()) {
+					$throwPoints[] = ThrowPoint::createExplicit($scope, new ObjectType(TypeError::class), $assignedExpr, false);
+				}
 			} else {
 				// fallback
 				$assignedExprType = $scope->getType($assignedExpr);
 				$nodeCallback(new PropertyAssignNode($var, $assignedExpr, $isAssignOp), $scope);
 				$scope = $scope->assignExpression($var, $assignedExprType);
+				// simulate dynamic property assign by __set to get throw points
+				if (!$propertyHolderType->hasMethod('__set')->no()) {
+					$throwPoints = array_merge($throwPoints, $this->processExprNode(
+						new MethodCall($var->var, '__set'),
+						$scope,
+						static function (): void {
+						},
+						$context,
+					)->getThrowPoints());
+				}
 			}
 
 		} elseif ($var instanceof Expr\StaticPropertyFetch) {

tests/PHPStan/Rules/Exceptions/CatchWithUnthrownExceptionRuleTest.php

@@ -215,4 +215,38 @@ public function testBug6262(): void
 		$this->analyse([__DIR__ . '/data/bug-6262.php'], []);
 	}
 
+	public function testBug6256(): void
+	{
+		if (PHP_VERSION_ID < 70400) {
+			self::markTestSkipped('Test requires PHP 7.4.');
+		}
+
+		$this->analyse([__DIR__ . '/data/bug-6256.php'], [
+			[
+				'Dead catch - TypeError is never thrown in the try block.',
+				25,
+			],
+			[
+				'Dead catch - TypeError is never thrown in the try block.',
+				31,
+			],
+			[
+				'Dead catch - TypeError is never thrown in the try block.',
+				45,
+			],
+			[
+				'Dead catch - Exception is never thrown in the try block.',
+				57,
+			],
+			[
+				'Dead catch - Throwable is never thrown in the try block.',
+				63,
+			],
+			[
+				'Dead catch - Exception is never thrown in the try block.',
+				100,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Exceptions/data/bug-6256.php

@@ -0,0 +1,115 @@
+<?php // lint >= 7.4
+
+namespace Bug6256;
+
+use Exception;
+
+final class A
+{
+	public int $integerType = 1;
+	public $mixedType;
+	public string $stringType;
+	/** @var string|int */
+	public $stringOrIntType;
+
+	function doFoo()
+	{
+		try {
+			$this->integerType = "string";
+		} catch (\TypeError $e) {
+			// not dead
+		}
+
+		try {
+			$this->mixedType = "string";
+		} catch (\TypeError $e) {
+			// dead
+		}
+
+		try {
+			$this->stringType = "string";
+		} catch (\TypeError $e) {
+			// dead
+		}
+
+		/** @var string|int $intOrString */
+		$intOrString = '';
+		try {
+			$this->integerType = $intOrString;
+		} catch (\TypeError $e) {
+			// not dead
+		}
+
+		try {
+			$this->stringOrIntType = 1;
+		} catch (\TypeError $e) {
+			// dead
+		}
+
+		try {
+			$this->integerType = "string";
+		} catch (\Error $e) {
+			// not dead
+		}
+
+		try {
+			$this->integerType = "string";
+		} catch (\Exception $e) {
+			// dead
+		}
+
+		try {
+			$this->dynamicProperty = 1;
+		} catch (\Throwable $e) {
+			// dead
+		}
+	}
+}
+
+final class B {
+
+	/**
+	 * @throws Exception
+	 */
+	public function __set(string $name, $value)
+	{
+		throw new Exception();
+	}
+
+	function doFoo()
+	{
+		try {
+			$this->dynamicProperty = "string";
+		} catch (\Exception $e) {
+			// not dead
+		}
+	}
+}
+
+final class C {
+
+	/**
+	 * @throws void
+	 */
+	public function __set(string $name, $value) {}
+
+	function doFoo()
+	{
+		try {
+			$this->dynamicProperty = "string";
+		} catch (\Exception $e) {
+			// dead
+		}
+	}
+}
+
+class D {
+	function doFoo()
+	{
+		try {
+			$this->dynamicProperty = "string";
+		} catch (\Exception $e) {
+			// not dead because class is not final
+		}
+	}
+}