Fixes #2567 -- optimize generated code with an existential is directly negated.

lib/coffee-script/nodes.js

@@ -2258,10 +2258,14 @@
 
     Op.prototype.compileUnary = function(o) {
       var op, parts, plusMinus;
+      parts = [op = this.operator];
+      if (op === '!' && this.first instanceof Existence) {
+        this.first.negated = !this.first.negated;
+        return this.first.compile(o);
+      }
       if (o.level >= LEVEL_ACCESS) {
         return (new Parens(this)).compile(o);
       }
-      parts = [op = this.operator];
       plusMinus = op === '+' || op === '-';
       if ((op === 'new' || op === 'typeof' || op === 'delete') || plusMinus && this.first instanceof Op && this.first.operator === op) {
         parts.push(' ');

src/nodes.coffee

@@ -1530,9 +1530,12 @@ exports.Op = class Op extends Base
 
   # Compile a unary **Op**.
   compileUnary: (o) ->
+    parts = [op = @operator]
+    if op is '!' and @first instanceof Existence
+      @first.negated = not @first.negated
+      return @first.compile o
     if o.level >= LEVEL_ACCESS
       return (new Parens this).compile o
-    parts = [op = @operator]
     plusMinus = op in ['+', '-']
     parts.push ' ' if op in ['new', 'typeof', 'delete'] or
                       plusMinus and @first instanceof Op and @first.operator is op

test/operators.coffee

@@ -281,4 +281,10 @@ test "#2197: Existential existential double trouble", ->
   func = -> counter++
   func()? ? 100
   eq counter, 1
+  
+test "#2567: Optimization of negated existential produces correct result", ->
+  a = 1
+  ok !(!a?)
+  ok !b?
+