Autowiring does not support union type and default value.

Not sure how you would autowire an intersection type, because we wire by interface name it can't be two at once? But union types should be possible, we should inject the first match I suppose.

If the Drupal Core provided aliases for the serializer it would look something like this:

services:
  serializer:
    class: Symfony\Component\Serializer\Serializer
    arguments: [{  }, {  }]
  Symfony\Component\Serializer\Normalizer\SerializerInterface: '@serializer'
  Symfony\Component\Serializer\Normalizer\DenormalizerInterface: '@serializer'
  Symfony\Component\Serializer\Normalizer\NormalizerInterface: '@serializer'

Assumed controller constructor:

public function __construct(
    protected NormalizerInterface&DenormalizerInterface $normalizer,
) {}

In this case, we can check if all interfaces of a intersection type refer to the same service. If any of the interfaces is not an alias or refers to another service throw an AutowiringFailedException.

For a union type, we can iterate over all types until the first service found.

What do you think about it?

I think that's pretty edge case and I still don't see an immediate use for it (we don't appear to have any constructors that would require this right now), so it depends how much overhead it adds.

I think this would require a recursive call, okay let's skip that case for now.

Could the issue summary be updated to include proposed solution to bug?

Not sure if this is an API change.

I added the missing sections but left TBD for sections I can't answer

Test coverage appears to be there https://git.drupalcode.org/issue/drupal-3417735/-/jobs/714169