Allow undefined variables passed into by-ref parameters only if the t…

…ype is nullable

src/Analyser/NodeScopeResolver.php

@@ -3797,6 +3797,7 @@ private function processArgs(
 		foreach ($args as $i => $arg) {
 			$assignByReference = false;
 			$parameter = null;
+			$parameterType = null;
 			if (isset($parameters) && $parametersAcceptor !== null) {
 				if (isset($parameters[$i])) {
 					$assignByReference = $parameters[$i]->passedByReference()->createsNewVariable();
@@ -3810,9 +3811,22 @@ private function processArgs(
 				}
 			}
 
+			$lookForUnset = false;
 			if ($assignByReference) {
 				if ($arg->value instanceof Variable) {
-					$scope = $this->lookForSetAllowedUndefinedExpressions($scope, $arg->value);
+					$isBuiltin = false;
+					if ($calleeReflection instanceof FunctionReflection && $calleeReflection->isBuiltin()) {
+						$isBuiltin = true;
+					} elseif ($calleeReflection instanceof ExtendedMethodReflection && $calleeReflection->getDeclaringClass()->isBuiltin()) {
+						$isBuiltin = true;
+					}
+					if (
+						$isBuiltin
+						|| ($parameterType === null || !$parameterType->isNull()->no())
+					) {
+						$scope = $this->lookForSetAllowedUndefinedExpressions($scope, $arg->value);
+						$lookForUnset = true;
+					}
 				}
 			}
 
@@ -3839,10 +3853,8 @@ private function processArgs(
 				$result = $this->processExprNode($stmt, $arg->value, $scopeToPass, $nodeCallback, $context->enterDeep());
 			}
 			$scope = $result->getScope();
-			if ($assignByReference) {
-				if ($arg->value instanceof Variable) {
-					$scope = $this->lookForUnsetAllowedUndefinedExpressions($scope, $arg->value);
-				}
+			if ($assignByReference && $lookForUnset) {
+				$scope = $this->lookForUnsetAllowedUndefinedExpressions($scope, $arg->value);
 			}
 
 			if ($calleeReflection !== null) {

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -1042,4 +1042,22 @@ public function testBug10418(): void
 		$this->analyse([__DIR__ . '/data/bug-10418.php'], []);
 	}
 
+	public function testPassByReferenceIntoNotNullable(): void
+	{
+		if (PHP_VERSION_ID < 80000) {
+			$this->markTestSkipped('Test requires PHP 8.0.');
+		}
+
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = true;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+		$this->analyse([__DIR__ . '/data/pass-by-reference-into-not-nullable.php'], [
+			[
+				'Undefined variable: $three',
+				32,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/pass-by-reference-into-not-nullable.php

@@ -0,0 +1,36 @@
+<?php // lint >= 8.0
+
+namespace PassByReferenceIntoNotNullable;
+
+class Foo
+{
+
+	public function doFooNoType(&$test)
+	{
+
+	}
+
+	public function doFooMixedType(mixed &$test)
+	{
+
+	}
+
+	public function doFooIntType(int &$test)
+	{
+
+	}
+
+	public function doFooNullableType(?int &$test)
+	{
+
+	}
+
+	public function test()
+	{
+		$this->doFooNoType($one);
+		$this->doFooMixedType($two);
+		$this->doFooIntType($three);
+		$this->doFooNullableType($four);
+	}
+
+}