Added failling test

while looking into `isset($arr[$i])` I realized that we have this untested case in `array_kex_exists`

Author: staabm

tests/PHPStan/Rules/Arrays/NonexistentOffsetInArrayDimFetchRuleTest.php

@@ -406,6 +406,16 @@ public function testBug7000(): void
 		]);
 	}
 
+	public function testBug7000b(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-7000b.php'], [
+			[
+				"Offset 'require'|'require-dev' might not exist on array{require?: array<string, string>, require-dev?: array<string, string>}.",
+				18,
+			],
+		]);
+	}
+
 	public function testBug6508(): void
 	{
 		$this->analyse([__DIR__ . '/data/bug-6508.php'], []);

tests/PHPStan/Rules/Arrays/data/bug-7000b.php

@@ -0,0 +1,22 @@
+<?php declare(strict_types = 1);
+
+namespace Bug7000b;
+
+use function array_key_exists;
+
+class Foo
+{
+	public function doBar(): void
+	{
+		/** @var array{require?: array<string, string>, require-dev?: array<string, string>} $composer */
+		$composer = array();
+		/** @var 'require'|'require-dev' $foo */
+		$foo = '';
+		foreach (array('require', 'require-dev') as $linkType) {
+			if (array_key_exists($linkType, $composer)) {
+				foreach ($composer[$linkType] as $x) {} // should not report error
+				foreach ($composer[$foo] as $x) {} // should report error. It can be $linkType = 'require', $foo = 'require-dev'
+			}
+		}
+	}
+}