Make cpdef methods overridable in Python classes with slots (which do not have a dict).

Closes #1771.

CHANGES.rst

@@ -28,6 +28,10 @@ Bugs fixed
 * The directive ``language_level=3`` did not apply to the first token in the
   source file.  (Github issue #2230)
 
+* Overriding cpdef methods did not work in Python subclasses with slots.
+  Note that this can have a performance impact on calls from Cython code.
+  (Github issue #1771)
+
 Other changes
 -------------
 

Cython/Compiler/Nodes.py

@@ -4338,7 +4338,9 @@ class OverrideCheckNode(StatNode):
         if self.py_func.is_module_scope:
             code.putln("else {")
         else:
-            code.putln("else if (unlikely(Py_TYPE(%s)->tp_dictoffset != 0)) {" % self_arg)
+            code.putln("else if (unlikely((Py_TYPE(%s)->tp_dictoffset != 0)"
+                       " || (Py_TYPE(%s)->tp_flags & (Py_TPFLAGS_IS_ABSTRACT | Py_TPFLAGS_HEAPTYPE)))) {" % (
+                self_arg, self_arg))
         func_node_temp = code.funcstate.allocate_temp(py_object_type, manage_ref=True)
         self.func_node.set_cname(func_node_temp)
         # need to get attribute manually--scope would return cdef method

tests/run/cpdef_method_override.pyx

+# mode: run
+# tag: cpdef
+# ticket: gh-1771
+
+cdef class BaseType:
+    """
+    >>> BaseType().callmeth()
+    BaseType.meth
+    """
+    def callmeth(self):
+        return self.meth()
+    cpdef meth(self):
+        print("BaseType.meth")
+
+
+class PyClass(BaseType):
+    """
+    >>> PyClass().callmeth()
+    PyClass.meth
+    """
+    def meth(self):
+        print("PyClass.meth")
+
+
+class PySlotsClass(BaseType):
+    """
+    >>> PySlotsClass().callmeth()
+    PySlotsClass.meth
+    """
+    __slots__ = []
+
+    def meth(self):
+        print("PySlotsClass.meth")