Fix phpstan/phpstan#6830: Variable inside loop might not be defined

- Modified intersectConditionalExpressions() in MutatingScope to match
  conditional expressions by their condition part when exact key match
  fails, then merge result type holders via and()
- Added conditionExpressionTypeHoldersEqual() helper method
- New regression test in tests/PHPStan/Rules/Variables/data/bug-6830b.php
- Root cause: conditional expression keys include result types, which
  change across loop iterations when variables are reassigned, causing
  the intersection to drop them entirely

Author: VincentLanglet

src/Analyser/MutatingScope.php

@@ -3432,11 +3432,29 @@ private function intersectConditionalExpressions(array $otherConditionalExpressi
 
 			$otherHolders = $otherConditionalExpressions[$exprString];
 			$intersectedHolders = [];
-			foreach ($holders as $key => $holder) {
-				if (!array_key_exists($key, $otherHolders)) {
+			foreach ($holders as $holder) {
+				$key = $holder->getKey();
+				if (array_key_exists($key, $otherHolders)) {
+					$intersectedHolders[$key] = $holder;
 					continue;
 				}
-				$intersectedHolders[$key] = $holder;
+
+				foreach ($otherHolders as $otherHolder) {
+					if (!$this->conditionExpressionTypeHoldersEqual(
+						$holder->getConditionExpressionTypeHolders(),
+						$otherHolder->getConditionExpressionTypeHolders(),
+					)) {
+						continue;
+					}
+
+					$mergedTypeHolder = $holder->getTypeHolder()->and($otherHolder->getTypeHolder());
+					$mergedHolder = new ConditionalExpressionHolder(
+						$holder->getConditionExpressionTypeHolders(),
+						$mergedTypeHolder,
+					);
+					$intersectedHolders[$mergedHolder->getKey()] = $mergedHolder;
+					break;
+				}
 			}
 
 			if (count($intersectedHolders) === 0) {
@@ -3449,6 +3467,28 @@ private function intersectConditionalExpressions(array $otherConditionalExpressi
 		return $newConditionalExpressions;
 	}
 
+	/**
+	 * @param array<string, ExpressionTypeHolder> $a
+	 * @param array<string, ExpressionTypeHolder> $b
+	 */
+	private function conditionExpressionTypeHoldersEqual(array $a, array $b): bool
+	{
+		if (count($a) !== count($b)) {
+			return false;
+		}
+
+		foreach ($a as $exprString => $holder) {
+			if (!array_key_exists($exprString, $b)) {
+				return false;
+			}
+			if (!$holder->equals($b[$exprString])) {
+				return false;
+			}
+		}
+
+		return true;
+	}
+
 	/**
 	 * @param array<string, ConditionalExpressionHolder[]> $newConditionalExpressions
 	 * @param array<string, ConditionalExpressionHolder[]> $existingConditionalExpressions

tests/PHPStan/Rules/Variables/DefinedVariableRuleTest.php

@@ -1393,6 +1393,16 @@ public function testBug6830(): void
 		$this->analyse([__DIR__ . '/data/bug-6830.php'], []);
 	}
 
+	public function testBug6830b(): void
+	{
+		$this->cliArgumentsVariablesRegistered = true;
+		$this->polluteScopeWithLoopInitialAssignments = false;
+		$this->checkMaybeUndefinedVariables = true;
+		$this->polluteScopeWithAlwaysIterableForeach = true;
+
+		$this->analyse([__DIR__ . '/data/bug-6830b.php'], []);
+	}
+
 	public function testBug14019(): void
 	{
 		$this->cliArgumentsVariablesRegistered = true;

tests/PHPStan/Rules/Variables/data/bug-6830b.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Bug6830b;
+
+function test(bool $do): void
+{
+	if ($do) {
+		$x = 9999;
+	}
+
+	foreach ([1, 2, 3] as $whatever) {
+		if ($do) {
+			if ($x) {
+				$x = 123;
+			}
+		}
+	}
+}