Add regression test for #13759

Closes phpstan/phpstan#13759

Author: phpstan-bot

tests/PHPStan/Analyser/nsrt/bug-13759.php

@@ -0,0 +1,32 @@
+<?php declare(strict_types = 1); // lint >= 8.0
+
+namespace Bug13759;
+
+use function PHPStan\Testing\assertType;
+
+class Test
+{
+	public function scenario(): void
+	{
+		$ints = [];
+		foreach (['a', 'b'] as $key) {
+			$ints[$key] = 1;
+		}
+		$ints['c'] = 1;
+
+		assertType("array{a?: 1, b?: 1, c: 1}", $ints);
+
+		foreach (['a'] as $key) {
+			$ints[$key] = $this->intToSomething($ints[$key]);
+		}
+
+		assertType("array{a: float|string, b?: 1, c: 1}", $ints);
+	}
+
+	/**
+	 * @return string|float
+	 */
+	protected function intToSomething(int $int): string|float {
+		return mt_rand(1, 2) ? (string)$int : (float)$int;
+	}
+}