Some thoughts for Crystal arguments default value and type inference

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2

That’s precisely the problem, @option isn’t nilable, but you’re allowed to pass a nil argument to the constructor. This in turn changes the error from “no overload” to what you see there because you’re trying to assign a variable that can be nilable to an ivar that cannot.

In this case, I’d do something like:

def initialize(option : YAML::Any? = nil)
  @option = option || YAML.parse("{}")
end

Related: [RFC] Undefined Type