Fix regression: refine guard overlap check in createConditionalExpressions()

The previous guard overlap check was too aggressive - it blocked creating
conditional expressions in both loops of createConditionalExpressions(),
including cases needed for variable certainty tracking.

The fix:
1. Only apply the guard overlap check in the first loop (type narrowing),
   not the second loop (variable certainty/existence)
2. Additionally require that the guarded expression exists with certainty
   Yes in the other branch before skipping - this ensures we only skip
   when both branches define the variable (ambiguous type case), not when
   a variable is only defined in one branch (certainty tracking case)

Adds regression test for the case reported by VincentLanglet where
$order should be certainly defined after two sequential if-blocks that
together cover all cases of a union type parameter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -3604,7 +3604,9 @@ private function createConditionalExpressions(
 
 			foreach ($variableTypeGuards as $guardExprString => $guardHolder) {
 				if (
-					array_key_exists($guardExprString, $theirExpressionTypes)
+					array_key_exists($exprString, $theirExpressionTypes)
+					&& $theirExpressionTypes[$exprString]->getCertainty()->yes()
+					&& array_key_exists($guardExprString, $theirExpressionTypes)
 					&& $theirExpressionTypes[$guardExprString]->getCertainty()->yes()
 					&& !$guardHolder->getType()->isSuperTypeOf($theirExpressionTypes[$guardExprString]->getType())->no()
 				) {
@@ -3621,13 +3623,6 @@ private function createConditionalExpressions(
 			}
 
 			foreach ($typeGuards as $guardExprString => $guardHolder) {
-				if (
-					array_key_exists($guardExprString, $theirExpressionTypes)
-					&& $theirExpressionTypes[$guardExprString]->getCertainty()->yes()
-					&& !$guardHolder->getType()->isSuperTypeOf($theirExpressionTypes[$guardExprString]->getType())->no()
-				) {
-					continue;
-				}
 				$conditionalExpression = new ConditionalExpressionHolder([$guardExprString => $guardHolder], new ExpressionTypeHolder($mergedExprTypeHolder->getExpr(), new ErrorType(), TrinaryLogic::createNo()));
 				$conditionalExpressions[$exprString][$conditionalExpression->getKey()] = $conditionalExpression;
 			}

tests/PHPStan/Analyser/nsrt/bug-14411-regression.php

@@ -0,0 +1,36 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace Bug14411Regression;
+
+use function PHPStan\Testing\assertType;
+use function PHPStan\Testing\assertVariableCertainty;
+use PHPStan\TrinaryLogic;
+
+interface OrderInterface {}
+
+class Event
+{
+	/** @return mixed */
+	public function getSubject()
+	{
+		return new \stdClass();
+	}
+}
+
+function getOrder(Event|OrderInterface $event): OrderInterface
+{
+	if ($event instanceof Event) {
+		$order = $event->getSubject();
+		assert($order instanceof OrderInterface);
+	}
+
+	if ($event instanceof OrderInterface) {
+		$order = $event;
+	}
+
+	assertVariableCertainty(TrinaryLogic::createYes(), $order);
+
+	return $order;
+}