Unify :root when extending.

doc-src/SASS_CHANGELOG.md

@@ -11,6 +11,8 @@
 
 * Extending a selector that contains a non-final pseudo-class no longer crashes.
 
+* When `@extending`, only a single `:root` element will be retained.
+
 ## 3.4.12 (13 February 2015)
 
 [Tagged on GitHub](https://github.com/sass/sass/releases/tag/3.4.12).

lib/sass/selector/sequence.rb

@@ -260,10 +260,22 @@ def subweave(seq1, seq2)
         return [seq1] if seq2.empty?
 
         seq1, seq2 = seq1.dup, seq2.dup
-        init = merge_initial_ops(seq1, seq2)
-        return unless init
-        fin = merge_final_ops(seq1, seq2)
-        return unless fin
+        return unless (init = merge_initial_ops(seq1, seq2))
+        return unless (fin = merge_final_ops(seq1, seq2))
+
+        # Make sure there's only one root selector in the output.
+        root1 = has_root?(seq1.first) && seq1.shift
+        root2 = has_root?(seq2.first) && seq2.shift
+        if root1 && root2
+          return unless (root = root1.unify(root2))
+          seq1.unshift root
+          seq2.unshift root
+        elsif root1
+          seq2.unshift root1
+        elsif root2
+          seq1.unshift root2
+        end
+
         seq1 = group_selectors(seq1)
         seq2 = group_selectors(seq2)
         lcs = Sass::Util.lcs(seq2, seq1) do |s1, s2|
@@ -274,6 +286,7 @@ def subweave(seq1, seq2)
         end
 
         diff = [[init]]
+
         until lcs.empty?
           diff << chunks(seq1, seq2) {|s| parent_superselector?(s.first, lcs.first)} << [lcs.shift]
           seq1.shift
@@ -613,6 +626,11 @@ def extended_not_expanded_to_s(extended_not_expanded)
           "(#{choices.join ', '})"
         end.join ' '
       end
+
+      def has_root?(sseq)
+        sseq.is_a?(SimpleSequence) &&
+          sseq.members.any? {|sel| sel.is_a?(Pseudo) && sel.normalized_name == "root"}
+      end
     end
   end
 end

test/sass/extend_test.rb

@@ -503,6 +503,16 @@ def test_multilayer_pseudoclass_extend
 SCSS
   end
 
+  def test_root_only_allowed_at_root
+    assert_extends(':root .foo', '.bar .baz {@extend .foo}',
+      ':root .foo, :root .bar .baz')
+    assert_extends('.foo:root .bar', '.baz:root .bang {@extend .bar}',
+      '.foo:root .bar, .baz.foo:root .bang')
+    assert_extends('html:root .bar', 'xml:root .bang {@extend .bar}', 'html:root .bar')
+    assert_extends('.foo:root > .bar .x', '.baz:root .bang .y {@extend .x}',
+      '.foo:root > .bar .x, .baz.foo:root > .bar .bang .y')
+  end
+
   def test_comma_extendee
     assert_equal <<CSS, render(<<SCSS)
 .foo, .baz {