#4617: Previously it was illegal to delete a name from the local

namespace if it occurs as a free variable in a nested block.  This limitation
of the compiler has been lifted, and a new opcode introduced (DELETE_DEREF).

This sample was valid in 2.6, but fails to compile in 3.x without this change::

   >>> def f():
   ...     def print_error():
   ...        print(e)
   ...     try:
   ...        something
   ...     except Exception as e:
   ...        print_error()
   ...        # implicit "del e" here


This sample has always been invalid in Python, and now works::

   >>> def outer(x):
   ...     def inner():
   ...        return x
   ...     inner()
   ...     del x

There is no need to bump the PYC magic number: the new opcode is used
for code that did not compile before.

Lib/test/test_scope.py

@@ -217,13 +217,6 @@ class ScopeTests(unittest.TestCase):
                     return f
             """)
 
-        check_syntax_error(self, """if 1:
-            def f(x):
-                def g():
-                    return x
-                del x # can't del name
-            """)
-
         check_syntax_error(self, """if 1:
             def f():
                 def g():
@@ -272,6 +265,28 @@ class ScopeTests(unittest.TestCase):
         self.assertRaises(UnboundLocalError, errorInOuter)
         self.assertRaises(NameError, errorInInner)
 
+    def testUnboundLocal_AfterDel(self):
+        # #4617: It is now legal to delete a cell variable.
+        # The following functions must obviously compile,
+        # and give the correct error when accessing the deleted name.
+        def errorInOuter():
+            y = 1
+            del y
+            print(y)
+            def inner():
+                return y
+
+        def errorInInner():
+            def inner():
+                return y
+            y = 1
+            del y
+            inner()
+
+        self.assertRaises(UnboundLocalError, errorInOuter)
+        self.assertRaises(NameError, errorInInner)
+
+    def testUnboundLocal_AugAssign(self):
         # test for bug #1501934: incorrect LOAD/STORE_GLOBAL generation
         exec("""if 1:
             global_x = 1