WP_User::set_role leaves user object without any caps when role does not change

[jammitch]

The first thing WP_User::set_role does is clears out the user's existing caps array. Then it checks to see if the passed-in role is the user's current role, and quits the function if so. This is great if the role actually changes. If it does not, this leaves the user with the same role, but without any caps whatsoever for the remainder of the current page.

Core WordPress code may not use the function in this way, but plugins may.

Workaround:
Plugins calling set_role should wrap the call in the same-role check found inside the function.

Fix:
Do the same-role check prior to unsetting the caps.

Confirmed:

add_action( 'admin_notices', function() {
	$user = new WP_User( 2 ); // 2 being the id of a user with the 'subscriber' role

	print_r($user->caps);

	$user->set_role( 'subscriber' );

	print_r($user->caps);
});

Replying to blueyed:

The version number is earliest version in which the bug is confirmed and shouldn't normally be changed on existing tickets.

This code was introduced in 3.1 for #14708.

This is small and useful

In 22418:

When using WP_User::set_role() to set the role, don't unset all caps when the requested role matches the current role.

Props scribu, jammitch
fixes #18932

​[UT1117]