Symfony version(s) affected: 5.0.7

Description
In case there is a comment before the first item of an unindented collection there is a parse error:

You cannot define a mapping item when in a sequence at line ..

For example, parsing this yaml will generate the error:

test:
  item1:
  # comment
  - a
  item2:
  - b

The error will occur if there is a new dictionary item after the first item (in this case item2). It will occur only for unindented collections (when the dashes are on the same indent level as the key)

How to reproduce

<?php

require_once __DIR__ . '/vendor/autoload.php';

use Symfony\Component\Yaml\Yaml;

$yaml = <<<END
test:
  item1:
  # comment
  - a
  item2:
  - b
END;

Yaml::parse($yaml);

Possible Solution

The problem is that due to the comment the parser regards this as a single embedded block:

  - a
  item2:
  - b

which is not a correct yaml block.

I have a fix for it; will create a pull request...