Only preserve vacuous conditional expressions during if/else branch merging

The preserveVacuousConditionalExpressions logic was being applied to all
mergeWith calls, including loop merging. In loop contexts, this incorrectly
preserved per-iteration branch correlations (e.g., "if $operators is
array{Operator}, then $operands is array{}") across iterations, causing
types to be over-narrowed (int → 0, non-empty-list<int> → array{int}).

Fix: add a $preserveVacuousConditionals parameter to mergeWith() (default
false) and only pass true from the if/elseif/else branch merge sites in
NodeScopeResolver.

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

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -3362,7 +3362,7 @@ public function isInFirstLevelStatement(): bool
 		return $this->inFirstLevelStatement;
 	}
 
-	public function mergeWith(?self $otherScope): self
+	public function mergeWith(?self $otherScope, bool $preserveVacuousConditionals = false): self
 	{
 		if ($otherScope === null || $this === $otherScope) {
 			return $this;
@@ -3372,16 +3372,18 @@ public function mergeWith(?self $otherScope): self
 
 		$mergedExpressionTypes = $this->mergeVariableHolders($ourExpressionTypes, $theirExpressionTypes);
 		$conditionalExpressions = $this->intersectConditionalExpressions($otherScope->conditionalExpressions);
-		$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
-			$conditionalExpressions,
-			$this->conditionalExpressions,
-			$theirExpressionTypes,
-		);
-		$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
-			$conditionalExpressions,
-			$otherScope->conditionalExpressions,
-			$ourExpressionTypes,
-		);
+		if ($preserveVacuousConditionals) {
+			$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
+				$conditionalExpressions,
+				$this->conditionalExpressions,
+				$theirExpressionTypes,
+			);
+			$conditionalExpressions = $this->preserveVacuousConditionalExpressions(
+				$conditionalExpressions,
+				$otherScope->conditionalExpressions,
+				$ourExpressionTypes,
+			);
+		}
 		$conditionalExpressions = $this->createConditionalExpressions(
 			$conditionalExpressions,
 			$ourExpressionTypes,

src/Analyser/NodeScopeResolver.php

@@ -1146,7 +1146,7 @@ public function processStmtNode(
 					$throwPoints = array_merge($throwPoints, $branchScopeStatementResult->getThrowPoints());
 					$impurePoints = array_merge($impurePoints, $branchScopeStatementResult->getImpurePoints());
 					$branchScope = $branchScopeStatementResult->getScope();
-					$finalScope = $branchScopeStatementResult->isAlwaysTerminating() ? $finalScope : $branchScope->mergeWith($finalScope);
+					$finalScope = $branchScopeStatementResult->isAlwaysTerminating() ? $finalScope : $branchScope->mergeWith($finalScope, true);
 					$alwaysTerminating = $alwaysTerminating && $branchScopeStatementResult->isAlwaysTerminating();
 					if (count($branchScopeStatementResult->getEndStatements()) > 0) {
 						$endStatements = array_merge($endStatements, $branchScopeStatementResult->getEndStatements());
@@ -1170,7 +1170,7 @@ public function processStmtNode(
 
 			if ($stmt->else === null) {
 				if (!$ifAlwaysTrue && !$lastElseIfConditionIsTrue) {
-					$finalScope = $scope->mergeWith($finalScope);
+					$finalScope = $scope->mergeWith($finalScope, true);
 					$alwaysTerminating = false;
 				}
 			} else {
@@ -1182,7 +1182,7 @@ public function processStmtNode(
 					$throwPoints = array_merge($throwPoints, $branchScopeStatementResult->getThrowPoints());
 					$impurePoints = array_merge($impurePoints, $branchScopeStatementResult->getImpurePoints());
 					$branchScope = $branchScopeStatementResult->getScope();
-					$finalScope = $branchScopeStatementResult->isAlwaysTerminating() ? $finalScope : $branchScope->mergeWith($finalScope);
+					$finalScope = $branchScopeStatementResult->isAlwaysTerminating() ? $finalScope : $branchScope->mergeWith($finalScope, true);
 					$alwaysTerminating = $alwaysTerminating && $branchScopeStatementResult->isAlwaysTerminating();
 					if (count($branchScopeStatementResult->getEndStatements()) > 0) {
 						$endStatements = array_merge($endStatements, $branchScopeStatementResult->getEndStatements());

tests/PHPStan/Analyser/nsrt/bug-13385b.php

@@ -31,13 +31,13 @@ public function calculate(array $children): int {
 					$value = $op->calculate($left, $right);
 
 					assertType(Operator::class, $op);
-					assertType('0', $left); // could be int
-					assertType('0', $right); // could be int
+					assertType('int', $left);
+					assertType('int', $right);
 					assertType('int', $value);
 
 					$operands[] = $value;
 
-					assertType('array{int}', $operands); // could be non-empty-list<int>
+					assertType('non-empty-list<int>', $operands);
 				}
 
 				$operators[] = $child;