Which features are not provided by the ClassLoader ?

regarding the explanations in your blog post, the difference between namespace prefix and class prefix for non-namespaced classes was an unnecessary complexity in the UniversalClassLoader. In both cases, it is just a prefix of the class name. this is why Composer dropped this distinction and then we dropped it also in our new implementation.
and for PSR-4 support, this is implemented by the Psr4ClassLoader in Symfony. The ClassLoader is only about PSR-0 (Composer decided to use a single class for both, with different setters instead)

Which features are not provided by the ClassLoader ?

With UniversalClassLoader, last call to registerNamespace() have prioritty, while with ClassLoader first call to addPrefix() have priority. This is a really different behavior (Adding vs Setting)

With UniversalClassLoader we have different registration for namsepace and prefix.
So an additional trailing \ is needed when using ClassLoader::addPrefix en ensure correct behavior.

And with UniversalClassLoader no duplicated path is possible, while with ClassLoader you can add various time the same prefix with the path (the reason of this PR)

P.S. I don't ask to drop deprecation of UniversalClassLoader, we can live with ClassLoader (once the differences are understood), I just like to fix this duplicate path / perf issue.

@remicollet technically, UniversalClassLoader still allows to have several times the same path for a prefix. It just does not care about it. The difference is that it overwrites the existing paths each time, so each call has to pass all paths for a given prefix rather than being able to add new ones without knowing the current state

@stof, so we agree UniversalClassLoader and ClassLoader are different. And this is not the goal of this PR to change this.

Any feedback on this PR ?

Thank you @remicollet.

merged into 2.3, which is still maintained.

Thanks a lot.