Add Version::NULL singleton

Library/Homebrew/test/test_versions.rb

@@ -30,6 +30,29 @@ def test_to_s
   end
 end
 
+class NullVersionTests < Homebrew::TestCase
+  def test_null_version_is_always_smaller
+    assert_operator Version::NULL, :<, version("1")
+  end
+
+  def test_null_version_is_never_greater
+    refute_operator Version::NULL, :>, version("0")
+  end
+
+  def test_null_version_is_not_equal_to_itself
+    refute_eql Version::NULL, Version::NULL
+  end
+
+  def test_null_version_creates_an_empty_string
+    assert_eql "", Version::NULL.to_s
+  end
+
+  def test_null_version_produces_nan_as_a_float
+    # Float::NAN is not equal to itself so compare object IDs
+    assert_eql Float::NAN.object_id, Version::NULL.to_f.object_id
+  end
+end
+
 class VersionNullTokenTests < Homebrew::TestCase
   def test_inspect
     assert_equal "#<Version::NullToken>", Version::NullToken.new.inspect

Library/Homebrew/version.rb

@@ -1,3 +1,5 @@
+require "version/null"
+
 class Version
   include Comparable
 
@@ -206,6 +208,10 @@ def head?
     false
   end
 
+  def null?
+    false
+  end
+
   def <=>(other)
     return unless other.is_a?(Version)
     return 0 if version == other.version

Library/Homebrew/version/null.rb

@@ -0,0 +1,38 @@
+class Version
+  NULL = Class.new do
+    include Comparable
+
+    def <=>(_other)
+      -1
+    end
+
+    def eql?(_other)
+      # Makes sure that the same instance of Version::NULL
+      # will never equal itself; normally Comparable#==
+      # will return true for this regardless of the return
+      # value of #<=>
+      false
+    end
+
+    def detected_from_url?
+      false
+    end
+
+    def head?
+      false
+    end
+
+    def null?
+      true
+    end
+
+    def to_f
+      Float::NAN
+    end
+
+    def to_s
+      ""
+    end
+    alias_method :to_str, :to_s
+  end.new
+end