what about a new path syntax to do this:

path: '/exports/news.{>_format?xml}' (note the?xml is already possible, it's about> here).

path: '/exports/news.{{_format}}' could also do the trick.

I like@ro0NL's idea to deal with this at the param syntax level.
I'd suggest using an! for that:/foo.{!bar} => this would enforce bar to have a default - and would always use it when generating URLs.
WDYT?

My original idea was to have a canonical URL generated.
Here the example has only one parameter but there could be more :

the_route:  path: /{section}.{_format}  requirements:    section: (index|news|authors)    _format: (html|xml|json)  defaults:    section: index    _format: html

Having the option on the parameter level gives more flexibility but IMO defeats the purpose of the canonical URL.

(BTW, the next step I had in mind was having the Router sending a Redirect when it matches a route on a non-canonical form.)

we can debate if redirecting non-canonical to canonical URLs is something that belongs to core 🤔

if so, that might also be triggered by e.g.canonical: true, which IMHO is a better semantic thenexplicit_defaults. In that case it would create a redirect with a default value for all parameters.

The generated URL is already a canonical one to me. I get you'd like it to have the suffix in it instead. It's not about canonical 1/0 to me, but about suffix 1/0, isn't it?

@chs2 WDYT about these proposals? Would you like to implement it?

I'd like to but I can't get my head around the param syntax especially with the XML/XSD.
I could go with "canonical: true" proposed by@ro0NL (instead of my "explicit_defaults").

@chs2 i dont think we need canonical:true|false, as@nicolas-grekas mentioned each configured route is/should be already a canonical one. So the options are:

• Handle the duplicate content redirect in code / at server level (htaccess) or another router

news_export:path:'/exports/news.{_format}'defaults:_format:xml

• Handle the duplicate content by another route definition

news_export_non_canonical:path:'/export/news'controller:Symfony\Bundle\FrameworkBundle\Controller\RedirectController::redirectActiondefaults:route:news_export_xmlnews_export_xml:path:'/exports/news.xml'

These are explicit configurations to show we dont needcanonical: true as a feature. The user should be responsible here.

If we look at option 2, we see our missing step: it cant specify the{_format} placeholder to be required (so it would need a route per format).

That's what we should solve by the proposed{!_format} syntax, so we can do

news_export_non_canonical:path:'/export/news'controller:Symfony\Bundle\FrameworkBundle\Controller\RedirectController::redirectActiondefaults:route:news_exportnews_export:path:'/exports/news.{!_format}'defaults:_format:xml

Due! syntax we would always require a format (given or default).

Thanks@chs2 for raising the point. I think the! syntax is what we should implement.
I created#29593 to keep track of the idea.
Closing here as this is quite different.
Help wanted to implement that!