Allow explicit nil to be serialized by enums

This commit uses an `UNSET` constant as the default `serialized_val` on
`T::Enum`. This allows the runtime to understand the difference between
an explicit `nil` and an unset default when serializing enum values.
Sorbet already understands statically that a serialized value can be of
type `NilClass`, but the runtime was serializing as a string
representation based on the constant name instead.

gems/sorbet-runtime/lib/types/enum.rb

@@ -273,8 +273,11 @@ def ===(other)
 
   ### Private implementation ###
 
+  UNSET = T.let(Module.new.freeze, Module)
+  private_constant :UNSET
+
   sig {params(serialized_val: SerializedVal).void}
-  def initialize(serialized_val=nil)
+  def initialize(serialized_val=UNSET)
     raise 'T::Enum is abstract' if self.class == T::Enum
     if !self.class.started_initializing?
       raise "Must instantiate all enum values of #{self.class} inside 'enums do'."
@@ -300,7 +303,7 @@ def initialize(serialized_val=nil)
   sig {params(const_name: Symbol).void}
   def _bind_name(const_name)
     @const_name = const_name
-    @serialized_val = const_to_serialized_val(const_name) if @serialized_val.nil?
+    @serialized_val = const_to_serialized_val(const_name) if @serialized_val.equal?(UNSET)
     freeze
   end
 

gems/sorbet-runtime/test/types/enum.rb

@@ -19,6 +19,7 @@ class CardSuitCustom < T::Enum
       SPADE = new('_spade_')
       DIAMOND = new('_diamond_')
       HEART = new('_heart_')
+      NONE = new(nil)
     end
   end
 
@@ -38,6 +39,7 @@ class CardSuitCustom < T::Enum
         assert_equal('_spade_', CardSuitCustom::SPADE.serialize)
         assert_equal('_diamond_', CardSuitCustom::DIAMOND.serialize)
         assert_equal('_heart_', CardSuitCustom::HEART.serialize)
+        assert_equal(nil, CardSuitCustom::NONE.serialize)
       end
     end
   end
@@ -361,10 +363,11 @@ class CustomSerializationEnum < T::Enum
           enums do
             new('foo')
             new
+            new(nil)
           end
         end
       end
-      assert_equal('Enum values must be assigned to constants: ["foo", nil]', ex.message)
+      assert_equal('Enum values must be assigned to constants: ["foo", T::Enum::UNSET, nil]', ex.message)
     end
   end