Fix incorrect type narrowing of superglobal with dependent types

- Extended conditional expression skip logic in MutatingScope to handle
  superglobal variables not present in the other branch's expression types
- Added exprContainsSuperGlobal() helper method to detect superglobal
  expressions for reuse across merge logic
- New regression test in tests/PHPStan/Rules/Variables/data/bug-14421.php
- Root cause: superglobals always exist but aren't tracked in expressionTypes
  unless narrowed, so the dependent type skip check failed to recognize them

Author: phpstan-bot

src/Analyser/MutatingScope.php

@@ -840,6 +840,20 @@ private function isGlobalVariable(string $variableName): bool
 		return in_array($variableName, self::SUPERGLOBAL_VARIABLES, true);
 	}
 
+	private function exprContainsSuperGlobal(Expr $expr): bool
+	{
+		$containsSuperGlobal = $expr->getAttribute(self::CONTAINS_SUPER_GLOBAL_ATTRIBUTE_NAME);
+		if ($containsSuperGlobal !== null) {
+			return $containsSuperGlobal;
+		}
+
+		$nodeFinder = new NodeFinder();
+		$containsSuperGlobal = $nodeFinder->findFirst($expr, fn (Node $node) => $node instanceof Variable && is_string($node->name) && $this->isGlobalVariable($node->name)) !== null;
+		$expr->setAttribute(self::CONTAINS_SUPER_GLOBAL_ATTRIBUTE_NAME, $containsSuperGlobal);
+
+		return $containsSuperGlobal;
+	}
+
 	/** @api */
 	public function hasConstant(Name $name): bool
 	{
@@ -3603,9 +3617,13 @@ private function createConditionalExpressions(
 			}
 
 			foreach ($variableTypeGuards as $guardExprString => $guardHolder) {
+				$exprExistsInTheirs = array_key_exists($exprString, $theirExpressionTypes)
+					&& $theirExpressionTypes[$exprString]->getCertainty()->yes();
+				if (!$exprExistsInTheirs) {
+					$exprExistsInTheirs = $this->exprContainsSuperGlobal($holder->getExpr());
+				}
 				if (
-					array_key_exists($exprString, $theirExpressionTypes)
-					&& $theirExpressionTypes[$exprString]->getCertainty()->yes()
+					$exprExistsInTheirs
 					&& array_key_exists($guardExprString, $theirExpressionTypes)
 					&& $theirExpressionTypes[$guardExprString]->getCertainty()->yes()
 					&& !$guardHolder->getType()->isSuperTypeOf($theirExpressionTypes[$guardExprString]->getType())->no()

tests/PHPStan/Rules/Variables/IssetRuleTest.php

@@ -562,4 +562,11 @@ public function testBug14393(): void
 		]);
 	}
 
+	public function testBug14421(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+
+		$this->analyse([__DIR__ . '/data/bug-14421.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-14421.php

@@ -0,0 +1,20 @@
+<?php declare(strict_types = 1);
+
+namespace Bug14421;
+
+/** @phpstan-impure */
+function get_optional_int(): ?int {
+	return random_int(0, 1) ? 42 : null;
+}
+
+if (isset($_SESSION['a'])) {
+    $b = $_SESSION['a'];
+}
+else {
+    $b = get_optional_int();
+}
+if ($b !== null) {
+ 	if (!isset($_SESSION['a'])) {
+		echo 'this is absolutely possible';
+	}
+}