When marking the template types of WeakMap, Psalm will notify you that WeakMap has no template params.

https://psalm.dev/r/b8cfb54105 (it appears WeakMap is not loaded on this environment - may be running 7.4):

ERROR: TooManyTemplateParams - src/test.php:4:11 - WeakMap<x, int> has too many template params, expecting 0 (see https://psalm.dev/184)
 * @param WeakMap<x,int> $wm

PHPStan reports a failure when template params are omitted for WeakMap, and a success when two template types are given.