[tenderlove]

Well, it does actually define locals. But, IIRC, it uses the symbol hash parsing productions, so the default values are required. The only way to specify required parameters (without the above hacks) is to do a traditional method definition:

    irb(main):001:0> def foo(bar, baz: Object.new); [bar, baz]; end
    => nil
    irb(main):002:0> foo(1)
    => [1, #<Object:0x007fbc39161ed8>]
    irb(main):003:0> foo(1, baz: 10)
    => [1, 10]
    irb(main):004:0> foo(baz: 10)
    ArgumentError: wrong number of arguments (0 for 1)
    	from (irb):1:in `foo'
    	from (irb):4
    	from /Users/aaron/.local/bin/irb:12:in `<main>'
    irb(main):005:0>