Add operator= to shared_ptr for assignments to base classes (GH-4185)

Cython/Includes/libcpp/memory.pxd

@@ -59,6 +59,7 @@ cdef extern from "<memory>" namespace "std" nogil:
         shared_ptr(shared_ptr[T]&, T*)
         shared_ptr(unique_ptr[T]&)
         #shared_ptr(weak_ptr[T]&) # Not Supported
+        shared_ptr[T]& operator=[Y](const shared_ptr[Y]& ptr)
 
         # Modifiers
         void reset()

tests/run/cpp_smart_ptr.pyx

@@ -81,6 +81,15 @@ cdef cppclass C(B):
 
 cdef shared_ptr[A] holding_subclass = shared_ptr[A](new C())
 
+
+def test_assignment_to_base_class():
+    """
+    >>> test_assignment_to_base_class()
+    """
+    cdef shared_ptr[C] derived = shared_ptr[C](new C())
+    cdef shared_ptr[A] base = derived
+
+
 def test_dynamic_pointer_cast():
     """
     >>> test_dynamic_pointer_cast()