Updated, thanks for the review.

A test could be added to RouteTest and RouteCompilerTest to cover this feature.

This is already covered by the current test to me.

The change in extractInlineDefaultsAndRequirements is not covered by tests, as you don't use an UTF-8 name on a parameter using inline config (try reverting that part of the diff and see that tests won't fail).
and changes in places computing $hasTrailingVar might need a test using a route using a trailing optional parameter using an UTF-8 name to be covered in a way preventing regression.

Tests added @stof

Thank you @nicolas-grekas.