Accept None as typed return value of dict.pop(), which was accidentally disallowed in 0.28.

Closes #2152.

CHANGES.rst

@@ -26,6 +26,9 @@ Bugs fixed
   in some cases where ``bytes`` would have been safe to infer.
   (Github issue #2153)
 
+* ``None`` was accidentally disallowed as typed return value of ``dict.pop()``.
+  (Github issue #2152)
+
 
 0.28 (2018-03-13)
 =================

Cython/Compiler/Optimize.py

@@ -3150,6 +3150,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
             node, function,
             "__Pyx_PyDict_Pop", self.PyDict_Pop_func_type,
             'pop', is_unbound_method, args,
+            may_return_none=True,
             utility_code=load_c_utility('py_dict_pop'))
 
     Pyx_PyInt_BinopInt_func_type = PyrexTypes.CFuncType(

tests/run/dict_pop.pyx

+# mode: run
+# tag: dict, pop, builtins
 
 cimport cython
 
@@ -32,3 +34,25 @@ def dict_pop_default(dict d, key, default):
     (20, {})
     """
     return d.pop(key, default), d
+
+
+cdef class MyType:
+    cdef public int i
+    def __init__(self, i):
+        self.i = i
+
+
+@cython.test_assert_path_exists("//SingleAssignmentNode//PythonCapiCallNode")
+@cython.test_fail_if_path_exists("//SingleAssignmentNode//AttributeNode")
+def dict_pop_default_typed(dict d, key, default):
+    """
+    >>> d = {1: MyType(2)}
+    >>> dict_pop_default_typed(d, 1, None)
+    2
+    >>> dict_pop_default_typed(d, 3, None)
+    >>> dict_pop_default_typed(d, 3, "default")  # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    TypeError: Cannot convert str to ...MyType
+    """
+    cdef MyType x = d.pop(key, default)
+    return x.i if x is not None else None