False positive unused variable when that variable is a reference

• PHPMD version: 2.10.2snapshot202107221018
• PHP Version: 8.0.13
• Installation type: phar file
• Operating System / Distribution & Version: Arch Linux

Current Behavior

PHPMD doesn't seem to have special handling for variables that are references (though it does seem to handle reference parameters correctly).

<?php

function func() {
	$a = 1;
	$b = &$a;
	$b = 2;
	print($a);
}

func();

$ ./phpmd.phar test.php text unusedcode
test.php:5	Avoid unused local variables such as '$b'.

However, $b is used.

#720 seems relevant but distinct.

Here's a less simplified example demonstrating why someone might use this:

<?php

class A {
	private static $cache = [];
	private static function &GetCache() {
		// Imagine that instead of returning the same cache, this instead
		// was shared logic to decide one of a number of caches to return
		return static::$cache;
	}

	public static function ClearCache() {
		$cache = &static::GetCache();

		$cache = [];
	}

	public static function InsertToCache($key, $value) {
		$cache = &static::GetCache();
		$cache[$key] = $value;
	}
}

PHPMD warns about ClearCache. (Curiously, it doesn't warn about InsertToCache- testing elsewhere, it seems like an array insertion is considered using a variable?)