Ruby自动将Hash扩展为关键字参数，而不需要双splat

Question

The below Ruby code results in: unknown keyword: a (ArgumentError) :

def test(x={}, y: true); end

test({a:1})

Why? I would expect this to happen with test(**{a:1}) , but I don't understand why my hash is being automagically expanded without the double splat.

Answer 1

Since x is optional, hash moves over to kwarg argument. Unspecified keywords raise error in that case:

def foo(name:)
  p name
end

foo # raises "ArgumentError: missing keyword: name" as expected
foo({name: 'Joe', age: 10}) # raises "ArgumentError: unknown keyword: age"

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Answer 2

I'd also find it a bug as it behaves quite inconsistently, only for hashes with keys of Symbol type:

test({a:1})  # raises ArgumentError
test({'a' => 1})  # nil, properly assigned to optional argument x
method(:test).parameters
=> [[:opt, :x], [:key, :y]]

You may pass both arguments and it starts to assign them properly, but this is not a solution.

test({a:1}, {y:false})  # nil

Any reason why this is not a bug but an expected behavior ?