arity.rb

def foo(x, y); end
def bar(x, y = 2); end
def baz(x = 1, y = 2); end

method(:foo).arity  #=> 2
method(:bar).arity  #=> -2
method(:baz).arity  #=> -1

def buz(x, y: 2); end
def pat(x: 1, y: 2); end

method(:buz).arity  #=> 1 (shouldn't this be -2 ?)
method(:pat).arity  #=> 0 (shouldn't this be -1 ?)