Avoid generating empty tp_new() functions if we can copy the function pointer...

Avoid generating empty tp_new() functions if we can copy the function pointer from the parent at init time.
See GH-1555.

Cython/Compiler/TypeSlots.py

@@ -366,24 +366,53 @@ class ConstructorSlot(InternalMethodSlot):
         InternalMethodSlot.__init__(self, slot_name, **kargs)
         self.method = method
 
-    def slot_code(self, scope):
-        entry = scope.lookup_here(self.method) if self.method else None
+    def _needs_own(self, scope):
         if (scope.parent_type.base_type
                 and not scope.has_pyobject_attrs
                 and not scope.has_memoryview_attrs
                 and not scope.has_cpp_class_attrs
-                and not (self.slot_name == 'tp_new' and scope.parent_type.vtabslot_cname)
-                and not (entry and entry.is_special)):
-            # if the type does not have object attributes, it can
-            # delegate GC methods to its parent - iff the parent
-            # functions are defined in the same module
+                and not (self.slot_name == 'tp_new' and scope.parent_type.vtabslot_cname)):
+            entry = scope.lookup_here(self.method) if self.method else None
+            if not (entry and entry.is_special):
+                return False
+        # Unless we can safely delegate to the parent, all types need a tp_new().
+        return True
+
+    def _parent_slot_function(self, scope):
         parent_type_scope = scope.parent_type.base_type.scope
         if scope.parent_scope is parent_type_scope.parent_scope:
             entry = scope.parent_scope.lookup_here(scope.parent_type.base_type.name)
             if entry.visibility != 'extern':
                 return self.slot_code(parent_type_scope)
+        return None
+
+    def slot_code(self, scope):
+        if not self._needs_own(scope):
+            # if the type does not have object attributes, it can
+            # delegate GC methods to its parent - iff the parent
+            # functions are defined in the same module
+            slot_code = self._parent_slot_function(scope)
+            return slot_code or '0'
         return InternalMethodSlot.slot_code(self, scope)
 
+    def generate_dynamic_init_code(self, scope, code):
+        if self.slot_code(scope) != '0':
+            return
+        # If we don't have our own slot function and don't know the
+        # parent function statically, copy it dynamically.
+        base_type = scope.parent_type.base_type
+        if base_type.is_extension_type and base_type.typeobj_cname:
+            src = '%s.%s' % (base_type.typeobj_cname, self.slot_name)
+        elif base_type.typeptr_cname:
+            src = '%s->%s' % (base_type.typeptr_cname, self.slot_name)
+        else:
+            return
+
+        code.putln("%s.%s = %s;" % (
+            scope.parent_type.typeobj_cname,
+            self.slot_name,
+            src))
+
 
 class SyntheticSlot(InternalMethodSlot):
     #  Type slot descriptor for a synthesized method which