Linking to a non-existent route should not throw an exception

This feels pretty major to me.

On catching RouteNotFoundException in the proper place(s), perhaps produce a '#' and carry on.

I would expect this to behave like links in Drupal 7 and earlier do. If code tries to link to a page that doesn't exist, the link itself should render fine. When the user clicks on the link, they should get a 404.

I agree we should not blow up, but on the other hand it would be interesting to know for module developers that such a case exist, as they should better introduce a module_exists() or similar. Does logging makes sense in that case?

One major difference to D6/D7 is that we no longer have the path, so as @cilefen wrote we can maybe like to <none>.

I can see the case for handling a missing route more gracefully, though technically a missing route is broken code.

I don't think linking to # is the correct way to handle this. In addition to the point @dawehner makes about logging, the magic behavior / silent non-failure seems incorrect. If the code links to a resource that doesn't exist, it seems to me like that link should be to a 404. This 404 request could log the fact that the route was missing and/or present this information on the 404 page.

Furthermore, something that appears to be a link but basically does nothing seems like a usability and accessibility problem.

Not novice at this point since there is not a consensus. :)

I like #5. +1 for a 404 - that is how the web is supposed to work.

To accomplish #5, we would have to create some kind of default 404 route in the system, and modify UrlGenerator::getRoute() to return it.

Could we not pass the string name of the bogus route as a parameter?

In other words, the URL would be /some/predetermined/404/path/bad_route_name.

Seems similar but unrelated to #2532490: Unrouted URLs break toolbar but are hidden by caching

There's more going on there, but #2532490: Unrouted URLs break toolbar but are hidden by caching is a similar problem space. (Different exception, similar underlying cause: Insufficient of handling of invalid links.) Cross-linking.

Yeah the question is whether we want to switch from exceptions to errors internally and otherwise return safe values like an empty string

I just ran into the same problem with permissions not realizing that 'access content' is now only provided when 'node' module is installed, which is not in 'testing' profile.

Therefore, I think that problem is pre-existing (In other spaces), we just have to realize that D8 is a lot more strict about links and urls.

Maybe a good idea would be to namespace (further new routes) as a convention with the module name?

Then it is pretty clear it is not a generic route ...

Just some thoughts ...

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Question: I assume user input urls and links are not affected as they go via a different code path, right?

We shipped with this, and I haven't heard anyone complain since.
I personally think we should mark it works as designed. If you don't own the route name you're linking to, your code should do whatever is necessary to not break if that route goes away (moduleExists, try/catch, ...)

It should at least fail gracefully. Even if that's a better description in a global exception catch so you get a useful error message.

This is what is happening:

The website encountered an unexpected error. Please try again later.

Symfony\Component\Routing\Exception\RouteNotFoundException: Route "foo" does not exist. in Drupal\Core\Routing\RouteProvider->getRouteByName() (line 191 of core/lib/Drupal/Core/Routing/RouteProvider.php).
Drupal\Core\Routing\UrlGenerator->getRoute('foo')

I'm not sure whether this is gracefully or not.

Graceful enough for me

Like so?

I'm wondering whether we could do this kind of error handling maybe further out in the layers ... Errors are something you an't catch in PHP 5.5/PHP 5.6. Maybe the URL generator is a better place?