Assertion error when paging a view using an entity autocomplete exposed filter

Problem/Motivation

- Create a view of nodes, add an exposed filter on the author (uid)
- Add enough nodes that it will need to use the pager
- Autocomplete the user that created the nodes above, submit the filter form
- Try to go to the next page, or any pager link
- Assertion Error (fatal if configured to do so e.g. settings.local.php config)

AssertionError: Tokens need to be valid Twig variables. in Drupal\Component\Assertion\Handle::Drupal\Component\Assertion\{closure}() (line 377 of core/modules/views/src/Plugin/views/PluginBase.php).

Proposed resolution

Tokens created from exposed input, like the autocomplete element (as it creates the nested structure with [['target_id' => VALUE]]) will generate tokens like arguments.author.0.target_id. In PluginBase::viewsTokenReplace() the tokens are validated with assertions. Most of the time this is fine, but when tokens contains dots, they are split and each piece is evaluated with the same assertion:

assert('preg_match(\'/^[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*$/\', $key) === 1', 'Tokens need to be valid Twig variables.');

So using the example token above, one of the parts would be '0' which would fail here. We should check is_numeric() || preg_match() but only in the case each $part is being checked.

Remaining tasks

Tests?

User interface changes

API changes

Data model changes