Explicitly passing nil does not set serialized value to nil

[jez]

Input

→ View on sorbet.run

# typed: true
require 'sorbet-runtime'

class MyEnum < T::Enum
  enums do
    X = new(nil)
  end
end

T.reveal_type(MyEnum::X.serialize) # => NilClass
p(MyEnum::X.serialize)             # => "x"

Observed output

editor.rb:10: Revealed type: NilClass https://srb.help/7014
    10 |T.reveal_type(MyEnum::X.serialize) # => NilClass
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  Got NilClass originating from:
    editor.rb:10:
    10 |T.reveal_type(MyEnum::X.serialize) # => NilClass
                      ^^^^^^^^^^^^^^^^^^^
Errors: 1

When this runs, it shows "x".

Expected behavior

It should be possible to specify the default serialized value of an enum to nil. Sorbet believes this is possible statically, and says the type is NilClass.

But at runtime, the value printed is "x", because sorbet-runtime does not distinguish between "explicitly passed nil for the serialized value" vs "passed no arguments."

---

Ideally, we fix this in sorbet-runtime using something like a private constant:

private_constant :UNSET
def initialize(serialized_value = UNSET)

and we check whether serialized_value is UNSET instead of whether it's nil to determine whether to infer a string serialized value.