Thank you @fancyweb.

I may have shot myself in the foot with #50415 😁 I have now upgraded to a version including this fix and am having issues with Swiftmailer configuration:

swiftmailer:
    encryption: '%env(string:default::MAILER_ENCRYPTION)%'

With this and MAILER_ENCRYPTION unset, the parameter now becomes an empty string ``. However, the Swiftmailer library does not like that as it only handles null in a special way, not an empty string, and throws

  [InvalidArgumentException]
  The  encryption is not supported

When I change the definition of the parameter to

swiftmailer:
    encryption: '%env(default::MAILER_ENCRYPTION)%'

which, as I understand this PR, should do what I expect it to (pass null as null), the container build crashes on type mismatch:

  [Symfony\Component\Config\Definition\Exception\InvalidTypeException]
  Invalid type for path "swiftmailer.mailers.default.encryption". Expected one of "bool", "int", "float", "string", but got one of "bool", "int", "float", "string", "array".

This is because the SwiftMailer configuration expects a scalar node but the result of the env-var-processing may be an array (though not in my particular case, I think).

This seems to be the case that you wrote about in the PR description

There's still no solution to have the "keep null as null" and "cast if not null" behavior.

It feels like there is a disconnect between what's possible with the parameter/yaml based configuration and what's possible to do with environment variables. One solution to the problem above would be to have a "nullable" set of prefixes, but I don't know if that's making things even more complicated.

Edit: this seems to do the trick in our code, at least:

swiftmailer:
    encryption: '%env(nullablestring:default::MAILER_ENCRYPTION)%'

<?php

use Symfony\Component\DependencyInjection\EnvVarProcessorInterface;
use Symfony\Component\DependencyInjection\Exception\RuntimeException;

final class NullableStringEnvProcessor implements EnvVarProcessorInterface
{
/**
     * {@inheritDoc}
     */
    public function getEnv(string $prefix, string $name, \Closure $getEnv)
    {
        if ($prefix === 'nullablestring') {
            $val = $getEnv($name);

            if ($val === null) {
                return null;
            }

            return (string) $val;
        }

        throw new RuntimeException(\sprintf('Unsupported env var prefix "%s" for env name "%s".', $prefix, $name));
    }

    /**
     * {@inheritDoc}
     */
    public static function getProvidedTypes()
    {
        return [
            'nullablestring' => 'string',
        ];
    }
}

Could you open a new issue with a reproducer and a full stack trace plz?

confirmed that this fix doesn't work unfortunately, empty stings instead of nulls are still passed to services.
problem noted above.

I tested it and indeed it doesn't work in the full framework context. I'll fix it again, for the last time I hope 😓