Only define classes if they don't exist - #277

The autoloader is responsible for requiring these files, and it will never be triggered at all if the class already exists.

Please describe how you get this error. It seems to indicate that you load the files in an unsupported way.

In a symfony project, attempting to use Normalizer without a use statement, like I said.

Please provide a full reproducer. The case you describe in one sentence is precisely the case that has been tested by the core team without any issue, meaning that you probably miss something.

How exactly do you use the class without a use statement?

I created a barebones project at:
https://github.com/someonewithpc/symfony-polyfill-test/blob/master/src/Kernel.php#L29

If you clone it and run php -S localhost:8080 public/index.php you should see an error like

Attempted to load class "Normalizer" from namespace "App".
Did you forget a "use" statement for "Symfony\Polyfill\Intl\Normalizer\Normalizer"?

(as an aside symfony server:start and bin/console server:start don't work, like suggested in the docs).

How exactly do you use the class without a use statement?

With it being in the global namespace, I'm not supposed to need to use it, so I just write $foo = Normalizer::normalize('foo');

@someonewithpc this error message tells me that your code is not in the global namespace but in the App namespace. To access a class of the global namespace, you must either add a use statement for it (to import it in your current namespace temporarily) or use a fully-qualified name (\Normalizer).
The rule to use classes of the global namespace is not different from the rule to use namespaced classes. Functions and constants have a fallback to the global namespace in their name resolution, but classes don't have it.

As @stof said you need to change your code to $foo = \Normalizer::normalize('foo'); if you don't want to use a use statement. I am closing here as that's not related to the polyfill.