Merge right scope of coalesce assign operator with scope before the e…

…xpression

src/Analyser/NodeScopeResolver.php

@@ -1959,10 +1959,7 @@ function (MutatingScope $scope) use ($expr, $nodeCallback, $context): Expression
 					$result = $this->processExprNode($expr->expr, $scope, $nodeCallback, $context->enterDeep());
 					if ($expr instanceof Expr\AssignOp\Coalesce) {
 						return new ExpressionResult(
-							$result->getScope()->mergeWith(
-								$this->processExprNode($expr->expr, $originalScope, static function (): void {
-								}, $context->enterDeep())->getScope(),
-							),
+							$result->getScope()->mergeWith($originalScope),
 							$result->hasYield(),
 							$result->getThrowPoints(),
 						);

tests/PHPStan/Rules/Variables/IssetRuleTest.php

@@ -450,4 +450,12 @@ public function testObjectShapes(): void
 		$this->analyse([__DIR__ . '/data/isset-object-shapes.php'], []);
 	}
 
+	public function testBug10151(): void
+	{
+		$this->treatPhpDocTypesAsCertain = true;
+		$this->strictUnnecessaryNullsafePropertyFetch = true;
+
+		$this->analyse([__DIR__ . '/data/bug-10151.php'], []);
+	}
+
 }

tests/PHPStan/Rules/Variables/data/bug-10151.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Bug10151;
+
+class Test
+{
+	/**
+	 * @var array<int>
+	 */
+	protected array $cache = [];
+
+	public function getCachedItemId (string $keyName): void
+	{
+		$result = $this->cache[$keyName] ??= ($newIndex = count($this->cache) + 1);
+
+		// WRONG ERROR: Variable $newIndex in isset() always exists and is not nullable.
+		if (isset($newIndex)) {
+			$this->recordNewCacheItem($keyName);
+		}
+	}
+
+	protected function recordNewCacheItem (string $keyName): void {
+		// ...
+	}
+}