We cannot replace the Symfony component by the PHP extension, as it would force people to install an uncommon PHP extension to use Symfony, which is a no-go (even the much more common mbstring can be an issue for some people, so my answer is not a wild guess).

And as the Symfony parser is turning mappings to associative PHP arrays, it cannot support using arrays as keys for instance. This is why support for complex mappings was never implemented until now.
We know that the parser does not implement the full spec (we even document this fact). We implement the most common features in config files (which is our focus for the component), and add more features based on feature requests, contributions, and the complexity it adds.
Btw, AFAIK you are the first one requesting it ever while the Symfony parser exists since years.

What should be possible is to detect the case of complex mappings and provide a more meaningful exception messages.

@BourotBenjamin possibly it should be possible to make a wrapper around the pecl extension which implements the interface needed by symfony. Then you could replace the yaml class yourself.

As my problem was created by an external service ( OneSky ) that exports complex yml event when you don't need complex keys, and I used a bundle to import and export files ( OpenClassrooms/OneSkyBundle ), I thought the best idea was to handle the problem in this bundle.

So I made a pull request to asks to install pecl_yaml with this bundle and made it laod the file by the pelc_yaml loader but export it with the SF2's yaml dumper.

@xabbuh do you think these improved exception messages would be easy to implement? Will you be able to do that for 3.3 (the feature freeze is in 32 days)? Thanks!

@javiereguiluz I am not sure, but I try to find a way to make that change.

see #22058

I think we need to deprecate the current behaviour first (see #22059).

The deprecated behaviour will be removed in #22770.

@BourotBenjamin I found the parsing Yaml file using PHP instead of c-extension badly inefficient.
Could you found a wrapper (class extension) over Symfony yam that uses the c-extension?