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 then explicit_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::redirectAction
  defaults:
    route: news_export_xml

news_export_xml:
  path: '/exports/news.xml'

These are explicit configurations to show we dont need canonical: 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::redirectAction
  defaults:
    route: news_export

news_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!