When using ~ as array key its interpreted as string "~" since i upgraded to symfony 3.3.
Before it was interpreted as null resp. "".

As it is in Yaml spec it should be interpreted as null even as key, i suppose:
http://www.yaml.org/type/null.html

Example:

my_service:
  class: SomeClass
  arguments:
    -
     ~: 'default'
     key: 'value'

Arguments results in:

array:2 [▼
"~" => "default"
"key" => "value"
]

Expected would have been:

array:2 [▼
"" => "default"
"key" => "value"
]

Is this intended behaviour?