Fix: reallowing tp_clear() in a subtype of an @no_gc_clear extension type...

Fix: reallowing tp_clear() in a subtype of an @no_gc_clear extension type generated an invalid C function call to the (non-existent) base type implementation.
Closes #2309.

CHANGES.rst

@@ -2,6 +2,17 @@
 Cython Changelog
 ================
 
+0.28.4 (2018-??-??)
+===================
+
+Bugs fixed
+----------
+
+* Reallowing ``tp_clear()`` in a subtype of an ``@no_gc_clear`` extension type
+  generated an invalid C function call to the (non-existent) base type implementation.
+  (Github issue #2309)
+
+
 0.28.3 (2018-05-27)
 ===================
 

Cython/Compiler/ModuleNode.py

@@ -1603,7 +1603,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         code.putln("}")
 
     def generate_clear_function(self, scope, code, cclass_entry):
-        tp_slot = TypeSlots.GCDependentSlot("tp_clear")
+        tp_slot = TypeSlots.get_slot_by_name("tp_clear")
         slot_func = scope.mangle_internal("tp_clear")
         base_type = scope.parent_type.base_type
         if tp_slot.slot_code(scope) != slot_func:

tests/run/no_gc_clear.pyx

@@ -56,6 +56,21 @@ cdef class DisableTpClear:
             pto.tp_clear(self)
 
 
+cdef class ReallowTpClear(DisableTpClear):
+    """
+    >>> import gc
+    >>> obj = ReallowTpClear()
+    >>> is_tp_clear_null(obj)
+    False
+
+    >>> obj.attr = obj  # create hard reference cycle
+    >>> del obj; _ignore = gc.collect()
+
+    # Problem: cannot really validate that the cycle was cleaned up without using weakrefs etc...
+    """
+    cdef public object attr
+
+
 def test_closure_without_clear(str x):
     """
     >>> c = test_closure_without_clear('abc')