Detect versions in tag specs

Closes Homebrew/homebrew#18300.
Closes Homebrew/homebrew#20891.

Library/Homebrew/cmd/audit.rb

@@ -276,7 +276,7 @@ class FormulaAuditor
         problem "Invalid or missing #{spec} version"
       else
         version_text = s.version unless s.version.detected_from_url?
-        version_url = Version.parse(s.url)
+        version_url = Version.detect(s.url, s.specs)
         if version_url.to_s == version_text.to_s && s.version.instance_of?(Version)
           problem "#{spec} version #{version_text} is redundant with version scanned from URL"
         end

Library/Homebrew/formula_support.rb

@@ -35,6 +35,14 @@ class SoftwareSpec
     raise e
   end
 
+  def detect_version(val)
+    case val
+    when nil    then Version.detect(url, specs)
+    when String then Version.new(val)
+    when Hash   then Version.new_with_scheme(*val.shift)
+    end
+  end
+
   # The methods that follow are used in the block-form DSL spec methods
   Checksum::TYPES.each do |cksum|
     class_eval <<-EOS, __FILE__, __LINE__ + 1
@@ -52,12 +60,7 @@ class SoftwareSpec
   end
 
   def version val=nil
-    @version ||=
+    @version ||= detect_version(val)
-      case val
-      when nil  then Version.parse(@url)
-      when Hash then Version.new_with_scheme(*val.shift)
-      else           Version.new(val)
-      end
   end
 
   def mirror val

Library/Homebrew/test/test_software_spec.rb

@@ -60,6 +60,12 @@ class SoftwareSpecTests < Test::Unit::TestCase
     assert_instance_of scheme, @spec.version
   end
 
+  def test_version_from_tag
+    @spec.url('http://foo.com/bar-1.0.tar.gz', :tag => 'v1.0.2')
+    assert_version_equal '1.0.2', @spec.version
+    assert @spec.version.detected_from_url?
+  end
+
   def test_mirrors
     assert_empty @spec.mirrors
     @spec.mirror('foo')

Library/Homebrew/version.rb

@@ -154,6 +154,14 @@ class Version
     end
   end
 
+  def self.detect(url, specs={})
+    if specs.has_key?(:tag)
+      new(specs[:tag][/((?:\d+\.)*\d+)/, 1], true)
+    else
+      parse(url)
+    end
+  end
+
   def initialize(val, detected=false)
     @version = val.to_s
     @detected_from_url = detected