allow 'not None' declaration on buffer arguments

Cython/Compiler/Nodes.py

@@ -2978,12 +2978,13 @@ class DefNode(FuncDefNode):
             arg.needs_conversion = 0
             arg.needs_type_test = 0
             arg.is_generic = 1
-            if arg.type.is_pyobject:
+            if arg.type.is_pyobject or arg.type.is_buffer:
                 if arg.or_none:
                     arg.accept_none = True
                 elif arg.not_none:
                     arg.accept_none = False
-                elif arg.type.is_extension_type or arg.type.is_builtin_type:
+                elif (arg.type.is_extension_type or arg.type.is_builtin_type
+                      or arg.type.is_buffer):
                     if arg.default and arg.default.constant_result is None:
                         # special case: def func(MyType obj = None)
                         arg.accept_none = True

tests/run/bufaccess.pyx

@@ -225,6 +225,20 @@ def as_argument(object[int] bufarg, int n):
         print bufarg[i],
     print 'END'
 
+@testcase
+def as_argument_not_none(object[int] bufarg not None):
+    """
+    >>> A = IntMockBuffer("A", range(6))
+    >>> as_argument_not_none(A)
+    acquired A
+    ACCEPTED
+    released A
+    >>> as_argument_not_none(None)
+    Traceback (most recent call last):
+    TypeError: Argument 'bufarg' must not be None
+    """
+    print 'ACCEPTED'
+
 @testcase
 def as_argument_defval(object[int] bufarg=IntMockBuffer('default', range(6)), int n=6):
     """