If you want use a named arguments, just change definiton:

services:
  App\Service\FirstClass:
    arguments:
      [$myString: 'Some string']

Any name/value that starts with special character needs to be quoted: '$myString': 'Some string'

In this case quoted: '$myString': 'Some string' not works.

But:

1> It's not what the documentation says
2> In the "standard" Symfony install, it works
3> In the flex install, if I define manually the 2 arguments like below, it works too:

services:
  App\Service\FirstClass:
    $secondClass: '@App\Service\SecondClass'
    $myString: 'Some string'

I don't think the problem come from the definition. It's just that with Flex install, when configuring an argument by name, other arguments are no longer autowired

@ByScripts : autowiring is not enabled in container.yml. Just in etc/packages/app.yaml. It's a per-file directive (_defaults).

Enable autowiring for all services declared inside the file (+ private by default & autoconfigure) by adding this to the container.yml file:

services:
    _defaults: { public: false, autowire: true, autoconfigure: true }
    # ...

or enable autowiring just for this service

Thank you.

Yet, autowiring works if I don't add the string param... why ?

Edit: Nevermind, I understand what @ogizanagi means. Thank you.

It's not autowiring, it the named arguments feature: #21383

So... now I understand. But I think it's a "strange" behavior.

The app is configured to autowire by default (packages/app.yaml), which is great, and somewhat logical.

But I think it'll be pretty common to need to configure a named parameter. And since for that we declare the service in container.yaml, it is no longer autowired.

Other solution would be to use the app.yaml file, but in this case the container.yaml become useless, and it would feel pretty "bad".

Last is what @ogizanagi proposes. Addind the _defaults to container.yaml. Which is the "best practice" I suppose.

But should not the container.yaml contains these _defaults by default when creating the project?

Because it feels inconsistent to me, no?

I clone your repo and try my solution with:

services:
  App\Service\FirstClass:
    arguments:
      -$myString: 'Some string'

And it works without any configuration. If you use named arguments just define it with array.

@shude It fixes the error when running bin/console, but it's not working, because what it does is passing an array as first argument.

I created a Command, then injected FirstClass in its constructor. When I run the command, I get this error:

Type error: Argument 1 passed to App\Service\FirstClass::__construct() must be an instance of App\Service\SecondClass, array given

Like @ogizanagi said, autowiring is not configured in this file (container.yaml), so it can't work without specifying each argument (or by adding _defaults).

Sorry, I was wrong. If manualy set autowire to true it works fine. i think, It should be written in the documentation

@ByScripts can you close here (and maybe open a doc PR or issue also?)

I've just opened symfony/symfony-docs#8242, we can close here.