Fix NativeMethodReflection::hasSideEffects()

src/Reflection/Native/NativeMethodReflection.php

@@ -10,6 +10,7 @@
 use PHPStan\Reflection\ReflectionProvider;
 use PHPStan\TrinaryLogic;
 use PHPStan\Type\Type;
+use PHPStan\Type\VoidType;
 
 class NativeMethodReflection implements MethodReflection
 {
@@ -48,6 +49,9 @@ public function __construct(
 		?Type $throwType
 	)
 	{
+		if ($reflection->getName() === 'setValue') {
+			var_dump('here');
+		}
 		$this->reflectionProvider = $reflectionProvider;
 		$this->declaringClass = $declaringClass;
 		$this->reflection = $reflection;
@@ -143,9 +147,29 @@ public function getThrowType(): ?Type
 
 	public function hasSideEffects(): TrinaryLogic
 	{
+		$name = strtolower($this->getName());
+		$isVoid = $this->isVoid();
+		if (
+			$name !== '__construct'
+			&& $isVoid
+		) {
+			return TrinaryLogic::createYes();
+		}
+
 		return $this->hasSideEffects;
 	}
 
+	private function isVoid(): bool
+	{
+		foreach ($this->variants as $variant) {
+			if (!$variant->getReturnType() instanceof VoidType) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
 	public function getDocComment(): ?string
 	{
 		if ($this->stubPhpDocString !== null) {

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -10334,6 +10334,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-3302.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-1511.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-4434.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-4231.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-4231.php

@@ -0,0 +1,27 @@
+<?php
+
+namespace Bug4231;
+
+use function PHPStan\Analyser\assertType;
+
+function (): void {
+	$property = new \ReflectionProperty($this, 'data');
+
+	if (is_null($property->getValue($this))) {
+		assertType('null', $property->getValue($this));
+		$property->setValue($this, 'Some value which is not null');
+		assertType('mixed', $property->getValue($this));
+	}
+};
+
+function (): void {
+	$property = new \ReflectionProperty($this, 'data');
+
+	if (is_null($property->getValue($this))) {
+		assertType('null', $property->getValue($this));
+		$property->setValue($this, 'Some value which is not null');
+
+		$value = $property->getValue($this);
+		assertType('mixed', $value);
+	}
+};