Why? Dumping a container to YAML or XML is perfectly valid for a non-compiled container. How is that a bug fix?

This is not a bug fix for sure.
Without compiler passes applied, the container is really in some intermediary "bootstrapping" state. For XML & Yaml, we don't care. But for the PhpDumper?
Anyway, I've no strong opinion on this, I'm just explaining the reasoning I had :-)

Updated. I think indeed it make no sense for YAML, etc.

Also no strong opinion on this (as compared to #19673) however it would simplify the PHP Dumper in 4.0 a lot.

Also tests are unclear about this, some call compile beforehand others dont.

it would simplify the PHP Dumper in 4.0 a lot.

I forgot to mention this argument and it was really the beginning of the idea: PhpDumper is already complex enough, there is no need to have all these !isFrozen code paths in the code. Thus I'm 👍 (when tests will be green of course).

There are some bottlenecks.. which im not sure how to solve;

Dumping an uncompiled ContainerBuilder is deprecated since version 3.3 and will not be supported anymore in 4.0. Compile the container beforehand: 7x
    2x in PhpDumperTest::testAddService from Symfony\Component\DependencyInjection\Tests\Dumper
    2x in PhpDumperTest::testDump from Symfony\Component\DependencyInjection\Tests\Dumper
    1x in PhpDumperTest::testDumpAutowireData from Symfony\Component\DependencyInjection\Tests\Dumper
    1x in PhpDumperTest::testAddParameters from Symfony\Component\DependencyInjection\Tests\Dumper
    1x in PhpDumperTest::testServicesWithAnonymousFactories from Symfony\Component\DependencyInjection\Tests\Dumper

Basically all service fixtures (Fixtures/php/) must be revised to compiled containers. ie. testAddService tests both variants rather explicitly (services9.php vs services9_compiled.php).

We need to separate these tests for legacy and add a services[0-9]_compiled.php variant for each one. Right?

Im still 👍 for deprecating this feature though.

PhpDumper is already complex enough, there is no need to have all these !isFrozen code paths in the code

Agree, this makes it a real burden to maintain.

👍 for doing this on PhpDumper only.

Looks like tests are passing :)

Thank you @ro0NL.