@extend !optional

extend_optional.diff

diff --git a/docs/extend.md b/docs/extend.md
index a0e037f..92b18c6 100644
--- a/docs/extend.md
+++ b/docs/extend.md
@@ -185,3 +185,20 @@ Yielding:
     }
 
 Note that if the selector is not extended, it won't be in the resulting CSS, so it's a powerful way to create a library of extendable code. While you can insert code through mixins, they would insert the same code every time you use them, while extending placeholders would give you compact output.
+
+## Optional extending
+
+Sometimes it might be usefull to be able to extend something that might
+exists or not depending on the context. You can suffix any selector by `optional` to achieve this:
+
+    $specialDesign
+      color: #FFF
+
+    .btn
+      @extend .design !optional, $specialDesign !optional
+
+Yielding:
+
+    .btn {
+      color: #fff;
+    }
diff --git a/lib/nodes/selector.js b/lib/nodes/selector.js
index 46db498..14f697c 100644
--- a/lib/nodes/selector.js
+++ b/lib/nodes/selector.js
@@ -23,6 +23,7 @@ var Selector = module.exports = function Selector(segs){
   Node.call(this);
   this.inherits = true;
   this.segments = segs;
+  this.optional = false;
 };
 
 /**
@@ -66,6 +67,7 @@ Selector.prototype.clone = function(parent){
   clone.column = this.column;
   clone.filename = this.filename;
   clone.inherits = this.inherits;
+  clone.optional = this.optional;
   clone.val = this.val;
   clone.segments = this.segments.map(function(node){ return node.clone(parent, clone); });
   return clone;
@@ -82,6 +84,7 @@ Selector.prototype.toJSON = function(){
   return {
     __type: 'Selector',
     inherits: this.inherits,
+    optional: this.optional,
     segments: this.segments,
     val: this.val,
     lineno: this.lineno,
diff --git a/lib/parser.js b/lib/parser.js
index efd024e..eb068e0 100644
--- a/lib/parser.js
+++ b/lib/parser.js
@@ -1073,12 +1073,26 @@ Parser.prototype = {
   extend: function(){
     var tok = this.expect('extend')
       , selectors = []
+      , sel
+      , tok
       , node
       , arr;
 
     do {
       arr = this.selectorParts();
-      if (arr.length) selectors.push(new nodes.Selector(arr));
+      if (arr.length) {
+        sel = new nodes.Selector(arr);
+
+        if ('!' == this.peek().type) {
+          tok = this.lookahead(2);
+          if ('ident' == tok.type && 'optional' == tok.val.name) {
+            this.skip(['!', 'ident']);
+            sel.optional = true;
+          }
+        }
+
+        selectors.push(sel);
+      }
     } while(this.accept(','));
 
     node = new nodes.Extend(selectors);
diff --git a/lib/visitor/evaluator.js b/lib/visitor/evaluator.js
index e2ea546..ee776fe 100644
--- a/lib/visitor/evaluator.js
+++ b/lib/visitor/evaluator.js
@@ -835,6 +835,7 @@ Evaluator.prototype.visitExtend = function(extend){
       // Cloning the selector for when we are in a loop and don't want it to affect
       // the selector nodes and cause the values to be different to expected
       selector: this.interpolate(selector.clone()).trim(),
+      optional: selector.optional,
       lineno: selector.lineno,
       column: selector.column
     });
diff --git a/lib/visitor/normalizer.js b/lib/visitor/normalizer.js
index e81b8eb..9ed77d4 100644
--- a/lib/visitor/normalizer.js
+++ b/lib/visitor/normalizer.js
@@ -378,6 +378,7 @@ Normalizer.prototype.extend = function(group, selectors){
   group.block.node.extends.forEach(function(extend){
     var groups = map[extend.selector];
     if (!groups) {
+      if (extend.optional) return;
       var err = new Error('Failed to @extend "' + extend.selector + '"');
       err.lineno = extend.lineno;
       err.column = extend.column;