Fix type of return value for WeakMap::offsetGet()

stubs/WeakReference.stub

@@ -26,4 +26,9 @@ final class WeakReference
  */
 final class WeakMap implements \ArrayAccess, \Countable, \IteratorAggregate
 {
+	/**
+	 * @param TKey $offset
+	 * @return TValue
+	 */
+	public function offsetGet($offset) {}
 }

tests/PHPStan/Analyser/nsrt/weakMap.php

@@ -0,0 +1,32 @@
+<?php // lint >= 8.0
+
+declare(strict_types = 1);
+
+namespace weakMap;
+
+use WeakMap;
+use function PHPStan\Testing\assertType;
+
+interface Foo {}
+interface Bar {}
+
+/**
+ * @param WeakMap<Foo, Bar> $weakMap
+ */
+function weakMapOffsetGetNotNullable(WeakMap $weakMap, Foo $foo): void
+{
+	$bar = $weakMap[$foo];
+
+	assertType(Bar::class, $bar);
+}
+
+
+/**
+ * @param WeakMap<Foo, Bar|null> $weakMap
+ */
+function weakMapOffsetGetNullable(WeakMap $weakMap, Foo $foo): void
+{
+	$bar = $weakMap[$foo];
+
+	assertType( 'weakMap\\Bar|null', $bar);
+}