Fix coalescing on an optional array offset

Author: rvanvelzen

src/Analyser/MutatingScope.php

@@ -1591,9 +1591,7 @@ private function resolveType(string $exprString, Expr $node): Type
 				return TypeCombinator::removeNull($leftType);
 			}
 
-			$rightType = $this->filterByFalseyValue(
-				new BinaryOp\NotIdentical($node->left, new ConstFetch(new Name('null'))),
-			)->getType($node->right);
+			$rightType = $this->filterByFalseyValue(new Expr\Isset_([$node->left]))->getType($node->right);
 
 			if ($result === null) {
 				return TypeCombinator::union(

tests/PHPStan/Analyser/NodeScopeResolverTest.php

@@ -1408,6 +1408,7 @@ public function dataFileAsserts(): iterable
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/conditional-vars.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-5961.php');
 		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10189.php');
+		yield from $this->gatherAssertTypes(__DIR__ . '/data/bug-10317.php');
 	}
 
 	/**

tests/PHPStan/Analyser/data/bug-10317.php

@@ -0,0 +1,14 @@
+<?php declare(strict_types = 1);
+
+namespace Bug10317;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @param array{'foo': string, 'bar'?: string} $array
+ */
+function myfunction(array $array): void
+{
+    $string = $array['bar'] ?? $array['foo'];
+    assertType('string', $string);
+}