Fix del builtin regression

An exception to the handling of builtins is they can not be
deleted.

Author: jayvdb

pyflakes/checker.py

@@ -864,10 +864,12 @@ def on_conditional_branch():
         if isinstance(self.scope, FunctionScope) and name in self.scope.globals:
             self.scope.globals.remove(name)
         else:
-            try:
-                del self.scope[name]
-            except KeyError:
+            binding = self.scope.get(name, None)
+            if not binding or isinstance(binding, Builtin):
                 self.report(messages.UndefinedName, node, name)
+                return
+
+        del self.scope[name]
 
     def handleChildren(self, tree, omit=None):
         for node in iter_child_nodes(tree, omit=omit):

pyflakes/test/test_undefined_names.py

@@ -371,6 +371,19 @@ def test_delUndefined(self):
         """Del an undefined name."""
         self.flakes('del a', m.UndefinedName)
 
+    def test_del_builtin(self):
+        """Del a builtin not possible."""
+        self.flakes('del range', m.UndefinedName)
+
+    def test_del_shadowed_builtin(self):
+        """
+        Del a function with same name as builtin.
+        """
+        self.flakes('''
+        def range(): pass
+        del range
+        ''')
+
     def test_delConditional(self):
         """
         Ignores conditional bindings deletion.