Fix selector pseudoclass extension.

Closes #1476
Closes #1481

doc-src/SASS_CHANGELOG.md

@@ -5,6 +5,8 @@
 
 ## 3.4.7 (Unreleased)
 
+* Stop crashing when extending selector pseudoclasses such as `:not()`.
+
 * `@extend` resolution and `is-superselector()` no longer consider `.foo > .baz`
   to be a superselector of `.foo > .bar > .baz`.
 

lib/sass/selector/simple_sequence.rb

@@ -89,7 +89,7 @@ module Sass
         resolved_members = @members.map do |sel|
           next sel unless sel.is_a?(Pseudo) && sel.selector
           sel.with_selector(sel.selector.resolve_parent_refs(super_cseq, !:implicit_parent))
-        end
+        end.flatten
 
         # Parent selector only appears as the first selector in the sequence
         unless (parent = resolved_members.first).is_a?(Parent)

test/sass/extend_test.rb

@@ -514,6 +514,18 @@ CSS
 SCSS
   end
 
+  def test_nested_pseudo_selectors
+    assert_equal <<CSS, render(<<SCSS)
+.foo .bar:not(.baz), .bang .bar:not(.baz) {
+  a: b; }
+CSS
+.foo {
+  .bar:not(.baz) {a: b}
+}
+.bang {@extend .foo}
+SCSS
+  end
+
   ## Long Extendees
 
   def test_long_extendee