3 files changed, 37 insertions, 3 deletions
diff --git a/Library/Homebrew/dependable.rb b/Library/Homebrew/dependable.rb
index 4a3d2de88..4060973c4 100644
--- a/Library/Homebrew/dependable.rb
+++ b/Library/Homebrew/dependable.rb
@@ -20,10 +20,16 @@ module Dependable
   end
 
   def required?
+    # FIXME: Should `required?` really imply `!build?`? And if so, why doesn't
+    #        any of `optional?` and `recommended?` equally imply `!build?`?
     !build? && !optional? && !recommended?
   end
 
+  def option_tags
+    tags - RESERVED_TAGS
+  end
+
   def options
-    Options.create(tags - RESERVED_TAGS)
+    Options.create(option_tags)
   end
 end
diff --git a/Library/Homebrew/dependency.rb b/Library/Homebrew/dependency.rb
index a3b9d7b51..0e03ae0ea 100644
--- a/Library/Homebrew/dependency.rb
+++ b/Library/Homebrew/dependency.rb
@@ -124,11 +124,39 @@ class Dependency
       all.map(&:name).uniq.map do |name|
         deps = grouped.fetch(name)
         dep  = deps.first
-        tags = deps.flat_map(&:tags).uniq
+        tags = merge_tags(deps)
         option_names = deps.flat_map(&:option_names).uniq
         dep.class.new(name, tags, dep.env_proc, option_names)
       end
     end
+
+    private
+
+    def merge_tags(deps)
+      options = deps.flat_map(&:option_tags).uniq
+      merge_necessity(deps) + merge_temporality(deps) + options
+    end
+
+    def merge_necessity(deps)
+      # Cannot use `deps.any?(&:required?)` here due to its definition.
+      if deps.any? { |dep| !dep.recommended? && !dep.optional? }
+        [] # Means required dependency.
+      elsif deps.any?(&:recommended?)
+        [:recommended]
+      else # deps.all?(&:optional?)
+        [:optional]
+      end
+    end
+
+    def merge_temporality(deps)
+      if deps.all?(&:build?)
+        [:build]
+      elsif deps.all?(&:run?)
+        [:run]
+      else
+        [] # Means both build and runtime dependency.
+      end
+    end
   end
 end
 
diff --git a/Library/Homebrew/test/test_dependency.rb b/Library/Homebrew/test/test_dependency.rb
index 4ba7033d9..16bad6507 100644
--- a/Library/Homebrew/test/test_dependency.rb
+++ b/Library/Homebrew/test/test_dependency.rb
@@ -48,7 +48,7 @@ class DependencyTests < Homebrew::TestCase
     assert_equal Dependency, merged.first.class
 
     foo_named_dep = merged.find {|d| d.name == "foo"}
-    assert_equal [:build, "bar"], foo_named_dep.tags
+    assert_equal ["bar"], foo_named_dep.tags
     assert_includes foo_named_dep.option_names, "foo"
     assert_includes foo_named_dep.option_names, "foo2"