Generate short wrappers for special methods like "__next__()" to adapt their...

Generate short wrappers for special methods like "__next__()" to adapt their signature for the PyCFunction entry in PyMethodDef.
Previously, their cast to a two-argument PyCFunction was incorrect.
See #2363.

Cython/Compiler/Code.py

@@ -2179,7 +2179,7 @@ class CCodeWriter(object):
         if entry.in_closure:
             self.put_giveref('Py_None')
 
-    def put_pymethoddef(self, entry, term, allow_skip=True):
+    def put_pymethoddef(self, entry, term, allow_skip=True, wrapper_code_writer=None):
         if entry.is_special or entry.name == '__getattribute__':
             if entry.name not in special_py_methods:
                 if entry.name == '__getattr__' and not self.globalstate.directives['fast_getattr']:
@@ -2189,16 +2189,14 @@ class CCodeWriter(object):
                 # that's better than ours.
                 elif allow_skip:
                     return
-        from .TypeSlots import method_coexist
-        if entry.doc:
-            doc_code = entry.doc_cname
-        else:
-            doc_code = 0
+
         method_flags = entry.signature.method_flags()
-        if method_flags:
+        if not method_flags:
+            return
         if entry.is_special:
-                method_flags += [method_coexist]
-            func_ptr = entry.func_cname
+            from . import TypeSlots
+            method_flags += [TypeSlots.method_coexist]
+        func_ptr = wrapper_code_writer.put_pymethoddef_wrapper(entry) if wrapper_code_writer else entry.func_cname
         # Add required casts, but try not to shadow real warnings.
         cast = '__Pyx_PyCFunctionFast' if 'METH_FASTCALL' in method_flags else 'PyCFunction'
         if 'METH_KEYWORDS' in method_flags:
@@ -2210,9 +2208,20 @@ class CCodeWriter(object):
                 entry.name,
                 func_ptr,
                 "|".join(method_flags),
-                    doc_code,
+                entry.doc_cname if entry.doc else '0',
                 term))
 
+    def put_pymethoddef_wrapper(self, entry):
+        func_cname = entry.func_cname
+        if entry.is_special:
+            method_flags = entry.signature.method_flags()
+            if method_flags and 'METH_NOARGS' in method_flags:
+                # Special NOARGS methods really take no arguments besides 'self', but PyCFunction expects one.
+                func_cname = Naming.method_wrapper_prefix + func_cname
+                self.putln("static PyObject *%s(PyObject *self, CYTHON_UNUSED PyObject *arg) {return %s(self);}" % (
+                    func_cname, entry.func_cname))
+        return func_cname
+
     # GIL methods
 
     def put_ensure_gil(self, declare_gilstate=True, variable=None):

Cython/Compiler/ModuleNode.py

@@ -2144,18 +2144,24 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         if env.is_c_class_scope and not env.pyfunc_entries:
             return
         binding = env.directives['binding']
+
         code.putln("")
+        wrapper_code_writer = code.insertion_point()
+
         code.putln(
             "static PyMethodDef %s[] = {" % (
                 env.method_table_cname))
         for entry in env.pyfunc_entries:
             if not entry.fused_cfunction and not (binding and entry.is_overridable):
-                code.put_pymethoddef(entry, ",")
+                code.put_pymethoddef(entry, ",", wrapper_code_writer=wrapper_code_writer)
         code.putln(
             "{0, 0, 0, 0}")
         code.putln(
             "};")
 
+        if wrapper_code_writer.getvalue():
+            wrapper_code_writer.putln("")
+
     def generate_dict_getter_function(self, scope, code):
         dict_attr = scope.lookup_here("__dict__")
         if not dict_attr or not dict_attr.is_variable:

Cython/Compiler/Naming.py

@@ -28,6 +28,7 @@ const_prefix      = pyrex_prefix + "k_"
 py_const_prefix   = pyrex_prefix + "kp_"
 label_prefix      = pyrex_prefix + "L"
 pymethdef_prefix  = pyrex_prefix + "mdef_"
+method_wrapper_prefix = pyrex_prefix + "specialmethod_"
 methtab_prefix    = pyrex_prefix + "methods_"
 memtab_prefix     = pyrex_prefix + "members_"
 objstruct_prefix  = pyrex_prefix + "obj_"