[Comparable] Add in Comparable::_CompareToZero

core/comparable.rbs

@@ -167,5 +167,13 @@ interface Comparable::_WithSpaceshipOperator
   # If `other` is less than `self`, it returns a positive Integer.
   # If no comparison is defined with `other` and `self`, it returns `nil`.
   #
-  def <=>: (untyped other) -> Integer?
+  def <=>: (untyped other) -> Comparable::_CompareToZero?
+end
+
+# This interface indicates a type is comparable against zero.
+#
+interface Comparable::_CompareToZero
+  def <: (0) -> boolish
+
+  def >: (0) -> boolish
 end

test/stdlib/Comparable_test.rb

@@ -3,11 +3,17 @@
 class ComparableTest < Test::Unit::TestCase
   include TestHelper
 
+  class ComparableWithZero
+    def initialize(value) = @value = value
+    def <(zero) = @value < zero
+    def >(zero) = @value > zero
+  end
+
   class Test
     include Comparable
 
     def <=>(other)
-      rand(2) - 1
+      ComparableWithZero.new rand(2) - 1
     end
   end