diff --git a/Lib/test/test_clinic.py b/Lib/test/test_clinic.py
index 0274edd384da43..8d2aced0b9caaf 100644
--- a/Lib/test/test_clinic.py
+++ b/Lib/test/test_clinic.py
@@ -15,6 +15,7 @@
 import re
 import sys
 import unittest
+from unittest import mock
 import warnings
 
 test_tools.skip_if_missing('clinic')
@@ -22,7 +23,8 @@
     import libclinic
     from libclinic import ClinicError, unspecified, NULL, fail
     from libclinic.converters import (
-        int_converter, object_converter, str_converter, self_converter)
+        int_converter, object_converter, str_converter, self_converter,
+        Py_ssize_t_converter)
     from libclinic.function import (
         Module, Class, Function, FunctionKind, Parameter,
         permute_optional_groups, permute_right_option_groups,
@@ -34,6 +36,7 @@
     from libclinic.block_parser import Block, BlockParser
     from libclinic.codegen import BlockPrinter, Destination
     from libclinic.dsl_parser import DSLParser
+    from libclinic.slots import SlotSpec, SLOTS
     from libclinic.cli import parse_file, Clinic
 
 
@@ -807,6 +810,112 @@ def test_var_keyword_non_dict(self):
         """
         self.expect_failure(block, err, lineno=4)
 
+    def test_slot_next(self):
+        block = """
+            /*[clinic input]
+            output everything block
+            class Foo "FooObject *" "&Foo_Type"
+            [clinic start generated code]*/
+            /*[clinic input]
+            Foo.__next__
+            [clinic start generated code]*/
+            /*[clinic input]
+            @critical_section
+            Foo.__next__ as foo_iternext
+            [clinic start generated code]*/
+            #ifdef CONDITION
+            /*[clinic input]
+            Foo.__next__ as foo_iternext_cond
+            [clinic start generated code]*/
+            #endif
+        """
+        generated = self.clinic.parse(dedent(block))
+        self.assertIn(dedent("""
+            static PyObject *
+            Foo_next(PyObject *self)
+            {
+                return Foo_next_impl((FooObject *)self);
+            }
+        """), generated)
+        self.assertIn(dedent("""
+            static PyObject *
+            foo_iternext(PyObject *self)
+            {
+                PyObject *return_value = NULL;
+
+                Py_BEGIN_CRITICAL_SECTION(self);
+                return_value = foo_iternext_impl((FooObject *)self);
+                Py_END_CRITICAL_SECTION();
+
+                return return_value;
+            }
+        """), generated)
+        self.assertIn("#if defined(CONDITION)\n\nstatic PyObject *\n"
+                      "foo_iternext_cond_impl(FooObject *self);", generated)
+        # A slot function is not stored in a PyMethodDef.
+        for text in "METHODDEF", "PyDoc_", "#ifndef":
+            self.assertNotIn(text, generated)
+
+    def test_slot_with_parameters_and_return_type(self):
+        # Slots which take arguments or return something else than an
+        # object are not supported yet; check the machinery with fake ones.
+        slots = {
+            '__contains__': SlotSpec('__contains__', 'int', 'int',
+                                     {(object_converter,): 'sq_contains'}),
+            '__bool__': SlotSpec('__bool__', 'int', 'int', {(): 'nb_bool'}),
+        }
+        block = """
+            /*[clinic input]
+            output everything block
+            class Foo "FooObject *" "&Foo_Type"
+            [clinic start generated code]*/
+            /*[clinic input]
+            Foo.__contains__
+                value: object
+                /
+            [clinic start generated code]*/
+            /*[clinic input]
+            @critical_section
+            Foo.__bool__
+            [clinic start generated code]*/
+        """
+        with mock.patch.dict(SLOTS, slots):
+            generated = self.clinic.parse(dedent(block))
+        self.assertIn(dedent("""
+            static int
+            Foo_contains(PyObject *self, PyObject *value)
+            {
+                return Foo_contains_impl((FooObject *)self, value);
+            }
+        """), generated)
+        self.assertIn(dedent("""
+            static int
+            Foo_bool(PyObject *self)
+            {
+                int return_value = -1;
+
+                Py_BEGIN_CRITICAL_SECTION(self);
+                return_value = Foo_bool_impl((FooObject *)self);
+                Py_END_CRITICAL_SECTION();
+
+                return return_value;
+            }
+        """), generated)
+
+    def test_slot_cannot_be_cloned(self):
+        block = """
+            /*[clinic input]
+            class Foo "FooObject *" "&Foo_Type"
+            [clinic start generated code]*/
+            /*[clinic input]
+            Foo.__next__
+            [clinic start generated code]*/
+            /*[clinic input]
+            Foo.other = Foo.__next__
+            [clinic start generated code]*/
+        """
+        self.expect_failure(block, "Type slot methods cannot be cloned", lineno=8)
+
     def test_getset_in_ifdef(self):
         block = """
             /*[clinic input]
@@ -3237,6 +3346,78 @@ class Foo "" ""
                                   f"a normal callable")
                 self.expect_failure(block, expected_error, lineno=3)
 
+    def test_slot_next(self):
+        function = self.parse_function("""
+            module foo
+            class Foo "FooObject *" ""
+            Foo.__next__
+        """, signatures_in_block=3, function_index=2)
+        self.assertIs(function.kind, FunctionKind.SLOT)
+        self.assertEqual(function.slot.name, "__next__")
+        self.assertEqual(function.c_basename, "Foo_next")
+        self.assertEqual(function.return_converter.type, "PyObject *")
+        self.assertEqual(list(function.parameters), ["self"])
+
+    def test_slot_parameters(self):
+        slot = SlotSpec('__contains__', 'int', 'int', {
+            (Py_ssize_t_converter,): 'sq_fake',
+            (object_converter,): 'sq_contains'})
+        block = """
+            module foo
+            class Foo "FooObject *" ""
+            Foo.__contains__
+                value: {converter}
+                /
+        """
+        errors = [
+            ("Foo.__contains__",
+             "'__contains__' must declare (Py_ssize_t) for sq_fake or "
+             "(object) for sq_contains; got ()"),
+            ("Foo.__contains__\n    value: int\n    /",
+             "'__contains__' must declare"),
+            ("Foo.__contains__\n    value: object",
+             "The parameters of '__contains__' must be positional-only"),
+            ("Foo.__contains__\n    value: object = None\n    /",
+             "The parameters of '__contains__' cannot have a default value"),
+        ]
+        with mock.patch.dict(SLOTS, {'__contains__': slot}):
+            for converter in "Py_ssize_t", "object":
+                with self.subTest(converter=converter):
+                    function = self.parse_function(
+                        block.format(converter=converter),
+                        signatures_in_block=3, function_index=2)
+                    self.assertIs(function.kind, FunctionKind.SLOT)
+                    self.assertEqual(function.return_converter.type, "int")
+                    self.assertEqual(list(function.parameters), ["self", "value"])
+            for body, err in errors:
+                with self.subTest(block=body):
+                    body = 'class Foo "FooObject *" "&Foo_Type"\n' + body
+                    self.expect_failure(body, err)
+
+    def test_slot_errors(self):
+        cases = [
+            ("module foo\nfoo.__next__",
+             "'__next__' must be a method!"),
+            ("@classmethod\nFoo.__next__",
+             "'__next__' must be a normal method"),
+            ("@getter\nFoo.__next__",
+             "'__next__' must be a normal method"),
+            ("@coexist\nFoo.__next__",
+             "@coexist cannot be applied to '__next__'"),
+            ("@vectorcall\nFoo.__next__",
+             "@vectorcall can only be used with __init__ and __new__"),
+            ("Foo.__next__\n    a: int",
+             "'__next__' methods cannot define parameters"),
+            ("Foo.__next__ -> int",
+             "'Foo.__next__' cannot define a return type"),
+            ("Foo.__next__\n\nReturn the next item.",
+             "'__next__' methods cannot have a docstring"),
+        ]
+        for block, err in cases:
+            with self.subTest(block=block):
+                block = 'class Foo "FooObject *" "&Foo_Type"\n' + block
+                self.expect_failure(block, err)
+
     def test_deleter_without_setter(self):
         block = """
             module foo
diff --git a/Misc/NEWS.d/next/Tools-Demos/2026-10-20-12-50-00.gh-issue-114258.slotNx.rst b/Misc/NEWS.d/next/Tools-Demos/2026-10-20-12-50-00.gh-issue-114258.slotNx.rst
new file mode 100644
index 00000000000000..afaeda209e1bd2
--- /dev/null
+++ b/Misc/NEWS.d/next/Tools-Demos/2026-10-20-12-50-00.gh-issue-114258.slotNx.rst
@@ -0,0 +1,3 @@
+Argument Clinic now supports the ``__next__`` type slot (``tp_iternext``),
+including ``@critical_section``.
+
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 34548b94385693..484f7bc0f9d2d4 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -22,8 +22,9 @@ module instead.
 /*[clinic input]
 module _csv
 class _csv.Dialect "DialectObj *" "clinic_state()->dialect_type"
+class _csv.Reader "ReaderObj *" "clinic_state()->reader_type"
 [clinic start generated code]*/
-/*[clinic end generated code: output=da39a3ee5e6b4b0d input=042fcc14c7d541cc]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=7bdf66dc7d68ed90]*/
 #define NOT_SET ((Py_UCS4)-1)
 #define EOL ((Py_UCS4)-2)
 
@@ -124,8 +125,6 @@ typedef struct {
     bool nonascii_special;
 } DialectObj;
 
-#include "clinic/_csv.c.h"
-
 typedef struct {
     PyObject_HEAD
 
@@ -161,6 +160,8 @@ typedef struct {
 #define _ReaderObj_CAST(op)     ((ReaderObj *)(op))
 #define _WriterObj_CAST(op)     ((WriterObj *)(op))
 
+#include "clinic/_csv.c.h"
+
 /*
  * DIALECT class
  */
@@ -1020,11 +1021,15 @@ parse_reset(ReaderObj *self)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+_csv.Reader.__next__ as Reader_iternext
+[clinic start generated code]*/
+
 static PyObject *
-Reader_iternext_lock_held(PyObject *op)
+Reader_iternext_impl(ReaderObj *self)
+/*[clinic end generated code: output=3a02b0254a76e1fc input=97c70a4e2fe621e4]*/
 {
-    ReaderObj *self = _ReaderObj_CAST(op);
-
     PyObject *fields = NULL;
     Py_UCS4 c;
     Py_ssize_t pos, linelen;
@@ -1094,16 +1099,6 @@ Reader_iternext_lock_held(PyObject *op)
     return fields;
 }
 
-static PyObject *
-Reader_iternext(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = Reader_iternext_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return result;
-}
-
 static void
 Reader_dealloc(PyObject *op)
 {
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index e725e863a0740a..2374a43ef74eab 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -768,13 +768,18 @@ _io_BytesIO_truncate_impl(bytesio *self, PyObject *size)
     return PyLong_FromSsize_t(new_size);
 }
 
+/*[clinic input]
+@critical_section
+_io.BytesIO.__next__ as bytesio_iternext
+[clinic start generated code]*/
+
 static PyObject *
-bytesio_iternext_lock_held(PyObject *op)
+bytesio_iternext_impl(bytesio *self)
+/*[clinic end generated code: output=f530bdee3a720d48 input=7fb3a763efe11a21]*/
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
 
     Py_ssize_t n;
-    bytesio *self = bytesio_CAST(op);
 
     CHECK_CLOSED(self);
 
@@ -786,16 +791,6 @@ bytesio_iternext_lock_held(PyObject *op)
     return read_bytes_lock_held(self, n);
 }
 
-static PyObject *
-bytesio_iternext(PyObject *op)
-{
-    PyObject *ret;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    ret = bytesio_iternext_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return ret;
-}
-
 /*[clinic input]
 @critical_section
 _io.BytesIO.seek
diff --git a/Modules/_io/clinic/bytesio.c.h b/Modules/_io/clinic/bytesio.c.h
index afdb60e506a2ce..b2305e35ca81e7 100644
--- a/Modules/_io/clinic/bytesio.c.h
+++ b/Modules/_io/clinic/bytesio.c.h
@@ -511,6 +511,21 @@ _io_BytesIO_truncate(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
     return return_value;
 }
 
+static PyObject *
+bytesio_iternext_impl(bytesio *self);
+
+static PyObject *
+bytesio_iternext(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = bytesio_iternext_impl((bytesio *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(_io_BytesIO_seek__doc__,
 "seek($self, pos, whence=0, /)\n"
 "--\n"
@@ -780,4 +795,4 @@ _io_BytesIO___sizeof__(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 #define _IO_BYTESIO_CLOSED_GETSETDEF {"closed", (getter)_io_BytesIO_closed_get, (setter)NULL, _io_BytesIO_closed__doc__},
 
-/*[clinic end generated code: output=a3dc7a416dae6ae9 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=a2bc96c358d6e240 input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/stringio.c.h b/Modules/_io/clinic/stringio.c.h
index 66b7a57cdf9333..bda87205e6bb71 100644
--- a/Modules/_io/clinic/stringio.c.h
+++ b/Modules/_io/clinic/stringio.c.h
@@ -135,6 +135,21 @@ _io_StringIO_readline(PyObject *self, PyObject *const *args, Py_ssize_t nargs)
     return return_value;
 }
 
+static PyObject *
+stringio_iternext_impl(stringio *self);
+
+static PyObject *
+stringio_iternext(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = stringio_iternext_impl((stringio *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(_io_StringIO_truncate__doc__,
 "truncate($self, pos=None, /)\n"
 "--\n"
@@ -539,4 +554,4 @@ _io_StringIO_newlines_get(PyObject *self, void *Py_UNUSED(context))
 
 #define _IO_STRINGIO_NEWLINES_GETSETDEF {"newlines", (getter)_io_StringIO_newlines_get, (setter)NULL, NULL},
 
-/*[clinic end generated code: output=4100afa5e4f295d7 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=38a0385529d9cbef input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/textio.c.h b/Modules/_io/clinic/textio.c.h
index 6d63d4706f609c..51e75d05061799 100644
--- a/Modules/_io/clinic/textio.c.h
+++ b/Modules/_io/clinic/textio.c.h
@@ -1096,6 +1096,21 @@ _io_TextIOWrapper_close(PyObject *self, PyObject *Py_UNUSED(ignored))
     return return_value;
 }
 
+static PyObject *
+textiowrapper_iternext_impl(textio *self);
+
+static PyObject *
+textiowrapper_iternext(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = textiowrapper_iternext_impl((textio *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 static PyObject *
 _io_TextIOWrapper_name_get_impl(textio *self);
 
@@ -1246,4 +1261,4 @@ _io_TextIOWrapper_buffer_get(PyObject *self, void *Py_UNUSED(context))
 
 #define _IO_TEXTIOWRAPPER_BUFFER_GETSETDEF {"buffer", (getter)_io_TextIOWrapper_buffer_get, (setter)NULL, NULL},
 
-/*[clinic end generated code: output=89de9ab341df9f66 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=f3dd3fff36cd2142 input=a9049054013a1b77]*/
diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 93a82bcc0c6e3e..e9a46167cff596 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -420,13 +420,18 @@ _io_StringIO_readline_impl(stringio *self, Py_ssize_t size)
     return _stringio_readline(self, size);
 }
 
+/*[clinic input]
+@critical_section
+_io.StringIO.__next__ as stringio_iternext
+[clinic start generated code]*/
+
 static PyObject *
-stringio_iternext_lock_held(PyObject *op)
+stringio_iternext_impl(stringio *self)
+/*[clinic end generated code: output=51eb3991b1c79c41 input=b6cf3e73fab6b42e]*/
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
 
     PyObject *line;
-    stringio *self = stringio_CAST(op);
 
     CHECK_INITIALIZED(self);
     CHECK_CLOSED(self);
@@ -438,7 +443,7 @@ stringio_iternext_lock_held(PyObject *op)
     }
     else {
         /* XXX is subclassing StringIO really supported? */
-        line = PyObject_CallMethodNoArgs(op, &_Py_ID(readline));
+        line = PyObject_CallMethodNoArgs((PyObject *)self, &_Py_ID(readline));
         if (line && !PyUnicode_Check(line)) {
             PyErr_Format(PyExc_OSError,
                          "readline() should have returned a str object, "
@@ -460,16 +465,6 @@ stringio_iternext_lock_held(PyObject *op)
     return line;
 }
 
-static PyObject *
-stringio_iternext(PyObject *op)
-{
-    PyObject *ret;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    ret = stringio_iternext_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return ret;
-}
-
 /*[clinic input]
 @critical_section
 _io.StringIO.truncate
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 3736179807f45d..f33ab46b10fd2e 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -3287,12 +3287,17 @@ _io_TextIOWrapper_close_impl(textio *self)
     }
 }
 
+/*[clinic input]
+@critical_section
+_io.TextIOWrapper.__next__ as textiowrapper_iternext
+[clinic start generated code]*/
+
 static PyObject *
-textiowrapper_iternext_lock_held(PyObject *op)
+textiowrapper_iternext_impl(textio *self)
+/*[clinic end generated code: output=f365c4287cd43a1f input=c5b4c79faf0c98cb]*/
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     PyObject *line;
-    textio *self = textio_CAST(op);
 
     CHECK_ATTACHED(self);
 
@@ -3302,7 +3307,7 @@ textiowrapper_iternext_lock_held(PyObject *op)
         line = _textiowrapper_readline(self, -1);
     }
     else {
-        line = PyObject_CallMethodNoArgs(op, &_Py_ID(readline));
+        line = PyObject_CallMethodNoArgs((PyObject *)self, &_Py_ID(readline));
         if (line && !PyUnicode_Check(line)) {
             PyErr_Format(PyExc_OSError,
                          "readline() should have returned a str object, "
@@ -3326,16 +3331,6 @@ textiowrapper_iternext_lock_held(PyObject *op)
     return line;
 }
 
-static PyObject *
-textiowrapper_iternext(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = textiowrapper_iternext_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return result;
-}
-
 /*[clinic input]
 @critical_section
 @getter
diff --git a/Modules/clinic/_csv.c.h b/Modules/clinic/_csv.c.h
index b25f30893224cd..df3bdbadf29aed 100644
--- a/Modules/clinic/_csv.c.h
+++ b/Modules/clinic/_csv.c.h
@@ -6,6 +6,7 @@ preserve
 #  include "pycore_gc.h"          // PyGC_Head
 #  include "pycore_runtime.h"     // _Py_ID()
 #endif
+#include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_modsupport.h"    // _PyArg_NoPositional()
 
 PyDoc_STRVAR(_csv_Dialect___reduce____doc__,
@@ -87,6 +88,21 @@ _csv_Dialect___replace__(PyObject *self, PyObject *args, PyObject *kwargs)
     return return_value;
 }
 
+static PyObject *
+Reader_iternext_impl(ReaderObj *self);
+
+static PyObject *
+Reader_iternext(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = Reader_iternext_impl((ReaderObj *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(_csv_reader__doc__,
 "reader($module, iterable, dialect=\'excel\', /, **fmtparams)\n"
 "--\n"
@@ -449,4 +465,4 @@ _csv_field_size_limit(PyObject *module, PyObject *const *args, Py_ssize_t nargs,
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=1a90b8a8ed82497d input=a9049054013a1b77]*/
+/*[clinic end generated code: output=7e3a92d524d88023 input=a9049054013a1b77]*/
diff --git a/Modules/clinic/itertoolsmodule.c.h b/Modules/clinic/itertoolsmodule.c.h
index d9917b47dc9037..d6365b46f04b26 100644
--- a/Modules/clinic/itertoolsmodule.c.h
+++ b/Modules/clinic/itertoolsmodule.c.h
@@ -7,6 +7,7 @@ preserve
 #  include "pycore_runtime.h"     // _Py_ID()
 #endif
 #include "pycore_abstract.h"      // _PyNumber_Index()
+#include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_modsupport.h"    // _PyArg_UnpackKeywords()
 
 PyDoc_STRVAR(batched_new__doc__,
@@ -508,6 +509,36 @@ itertools_chain_from_iterable(PyObject *type, PyObject *arg)
     return return_value;
 }
 
+static PyObject *
+chain_next_impl(chainobject *self);
+
+static PyObject *
+chain_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = chain_next_impl((chainobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
+static PyObject *
+product_next_impl(productobject *self);
+
+static PyObject *
+product_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = product_next_impl((productobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(itertools_combinations__doc__,
 "combinations(iterable, r)\n"
 "--\n"
@@ -586,6 +617,21 @@ itertools_combinations(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     return return_value;
 }
 
+static PyObject *
+combinations_next_impl(combinationsobject *self);
+
+static PyObject *
+combinations_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = combinations_next_impl((combinationsobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(itertools_combinations_with_replacement__doc__,
 "combinations_with_replacement(iterable, r)\n"
 "--\n"
@@ -665,6 +711,21 @@ itertools_combinations_with_replacement(PyTypeObject *type, PyObject *args, PyOb
     return return_value;
 }
 
+static PyObject *
+cwr_next_impl(cwrobject *self);
+
+static PyObject *
+cwr_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = cwr_next_impl((cwrobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(itertools_permutations__doc__,
 "permutations(iterable, r=None)\n"
 "--\n"
@@ -732,6 +793,21 @@ itertools_permutations(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     return return_value;
 }
 
+static PyObject *
+permutations_next_impl(permutationsobject *self);
+
+static PyObject *
+permutations_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = permutations_next_impl((permutationsobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(itertools_accumulate__doc__,
 "accumulate(iterable, func=None, *, initial=None)\n"
 "--\n"
@@ -808,6 +884,21 @@ itertools_accumulate(PyTypeObject *type, PyObject *args, PyObject *kwargs)
     return return_value;
 }
 
+static PyObject *
+accumulate_next_impl(accumulateobject *self);
+
+static PyObject *
+accumulate_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = accumulate_next_impl((accumulateobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+
 PyDoc_STRVAR(itertools_compress__doc__,
 "compress(data, selectors)\n"
 "--\n"
@@ -980,4 +1071,19 @@ itertools_count(PyTypeObject *type, PyObject *args, PyObject *kwargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=a34a31f60100e0ff input=a9049054013a1b77]*/
+
+static PyObject *
+zip_longest_next_impl(ziplongestobject *self);
+
+static PyObject *
+zip_longest_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = zip_longest_next_impl((ziplongestobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+/*[clinic end generated code: output=4b55088ac24582b7 input=a9049054013a1b77]*/
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index c0023c839ca7fe..636b28fa06ef2e 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -65,6 +65,16 @@ find_state_by_type(PyTypeObject *tp)
     return get_module_state(mod);
 }
 
+// The generated code (clinic/itertoolsmodule.c.h) needs the object types
+// of the type slots implemented with Argument Clinic.
+typedef struct chainobject chainobject;
+typedef struct productobject productobject;
+typedef struct combinationsobject combinationsobject;
+typedef struct cwrobject cwrobject;
+typedef struct permutationsobject permutationsobject;
+typedef struct accumulateobject accumulateobject;
+typedef struct ziplongestobject ziplongestobject;
+
 /*[clinic input]
 module itertools
 class itertools.groupby "groupbyobject *" "clinic_state()->groupby_type"
@@ -77,16 +87,18 @@ class itertools.dropwhile "dropwhileobject *" "clinic_state()->dropwhile_type"
 class itertools.takewhile "takewhileobject *" "clinic_state()->takewhile_type"
 class itertools.starmap "starmapobject *" "clinic_state()->starmap_type"
 class itertools.chain "chainobject *" "clinic_state()->chain_type"
+class itertools.product "productobject *" "clinic_state()->product_type"
 class itertools.combinations "combinationsobject *" "clinic_state()->combinations_type"
-class itertools.combinations_with_replacement "cwr_object *" "clinic_state()->cwr_type"
+class itertools.combinations_with_replacement "cwrobject *" "clinic_state()->cwr_type"
 class itertools.permutations "permutationsobject *" "clinic_state()->permutations_type"
 class itertools.accumulate "accumulateobject *" "clinic_state()->accumulate_type"
+class itertools.zip_longest "ziplongestobject *" "clinic_state()->ziplongest_type"
 class itertools.compress "compressobject *" "clinic_state()->compress_type"
 class itertools.filterfalse "filterfalseobject *" "clinic_state()->filterfalse_type"
 class itertools.count "countobject *" "clinic_state()->count_type"
 class itertools.pairwise "pairwiseobject *" "clinic_state()->pairwise_type"
 [clinic start generated code]*/
-/*[clinic end generated code: output=da39a3ee5e6b4b0d input=aa48fe4de9d4080f]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=1086fb2d308d75d2]*/
 
 #define clinic_state() (find_state_by_type(type))
 #define clinic_state_by_cls() (get_module_state_by_cls(base_tp))
@@ -1877,11 +1889,11 @@ static PyType_Spec starmap_spec = {
 
 /* chain object **************************************************************/
 
-typedef struct {
+struct chainobject {
     PyObject_HEAD
     PyObject *source;                   /* Iterator over input iterables */
     PyObject *active;                   /* Currently running input iterator */
-} chainobject;
+};
 
 #define chainobject_CAST(op)    ((chainobject *)(op))
 
@@ -1963,10 +1975,16 @@ chain_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
-static inline PyObject *
-chain_next_lock_held(PyObject *op)
+/*[clinic input]
+@critical_section
+itertools.chain.__next__ as chain_next
+[clinic start generated code]*/
+
+static PyObject *
+chain_next_impl(chainobject *self)
+/*[clinic end generated code: output=91e716f45def7fa9 input=e17098e9dc88706b]*/
 {
-    chainobject *lz = chainobject_CAST(op);
+    chainobject *lz = self;
     PyObject *item;
 
     /* lz->source is the iterator of iterables. If it's NULL, we've already
@@ -2002,16 +2020,6 @@ chain_next_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-chain_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = chain_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 PyDoc_STRVAR(chain_doc,
 "chain(*iterables)\n\
 --\n\
@@ -2055,13 +2063,13 @@ static PyType_Spec chain_spec = {
 
 /* product object ************************************************************/
 
-typedef struct {
+struct productobject {
     PyObject_HEAD
     PyObject *pools;        /* tuple of pool tuples */
     Py_ssize_t *indices;    /* one index per pool */
     PyObject *result;       /* most recently returned result tuple */
     int stopped;            /* set to 1 when the iterator is exhausted */
-} productobject;
+};
 
 #define productobject_CAST(op)  ((productobject *)(op))
 
@@ -2182,10 +2190,16 @@ product_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.product.__next__ as product_next
+[clinic start generated code]*/
+
 static PyObject *
-product_next_lock_held(PyObject *op)
+product_next_impl(productobject *self)
+/*[clinic end generated code: output=d20b8571608c2043 input=54266fe7ed4a857a]*/
 {
-    productobject *lz = productobject_CAST(op);
+    productobject *lz = self;
     PyObject *pool;
     PyObject *elem;
     PyObject *oldelem;
@@ -2269,16 +2283,6 @@ product_next_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-product_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = product_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 static PyMethodDef product_methods[] = {
     {"__sizeof__", product_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */
@@ -2323,14 +2327,14 @@ static PyType_Spec product_spec = {
 
 /* combinations object *******************************************************/
 
-typedef struct {
+struct combinationsobject {
     PyObject_HEAD
     PyObject *pool;         /* input converted to a tuple */
     Py_ssize_t *indices;    /* one index per result element */
     PyObject *result;       /* most recently returned result tuple */
     Py_ssize_t r;           /* size of result tuple */
     int stopped;            /* set to 1 when the iterator is exhausted */
-} combinationsobject;
+};
 
 #define combinationsobject_CAST(op) ((combinationsobject *)(op))
 
@@ -2421,10 +2425,16 @@ combinations_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.combinations.__next__ as combinations_next
+[clinic start generated code]*/
+
 static PyObject *
-combinations_next_lock_held(PyObject *op)
+combinations_next_impl(combinationsobject *self)
+/*[clinic end generated code: output=19965c1a1f4b8f2a input=93000f6ec48380d7]*/
 {
-    combinationsobject *co = combinationsobject_CAST(op);
+    combinationsobject *co = self;
     PyObject *elem;
     PyObject *oldelem;
     PyObject *pool = co->pool;
@@ -2507,16 +2517,6 @@ combinations_next_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-combinations_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = combinations_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 static PyMethodDef combinations_methods[] = {
     {"__sizeof__", combinations_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */
@@ -2572,14 +2572,14 @@ static PyType_Spec combinations_spec = {
                 if sorted(indices) == list(indices):
                     yield tuple(pool[i] for i in indices)
 */
-typedef struct {
+struct cwrobject {
     PyObject_HEAD
     PyObject *pool;         /* input converted to a tuple */
     Py_ssize_t *indices;    /* one index per result element */
     PyObject *result;       /* most recently returned result tuple */
     Py_ssize_t r;           /* size of result tuple */
     int stopped;            /* set to 1 when the cwr iterator is exhausted */
-} cwrobject;
+};
 
 #define cwrobject_CAST(op)  ((cwrobject *)(op))
 
@@ -2673,10 +2673,16 @@ cwr_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.combinations_with_replacement.__next__ as cwr_next
+[clinic start generated code]*/
+
 static PyObject *
-cwr_next_lock_held(PyObject *op)
+cwr_next_impl(cwrobject *self)
+/*[clinic end generated code: output=760ddcfbf905ee37 input=d9ed46a28a5bb29a]*/
 {
-    cwrobject *co = cwrobject_CAST(op);
+    cwrobject *co = self;
     PyObject *elem;
     PyObject *oldelem;
     PyObject *pool = co->pool;
@@ -2753,16 +2759,6 @@ cwr_next_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-cwr_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = cwr_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 static PyMethodDef cwr_methods[] = {
     {"__sizeof__", cwr_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */
@@ -2818,7 +2814,7 @@ def permutations(iterable, r=None):
             return
 */
 
-typedef struct {
+struct permutationsobject {
     PyObject_HEAD
     PyObject *pool;         /* input converted to a tuple */
     Py_ssize_t *indices;    /* one index per element in the pool */
@@ -2826,7 +2822,7 @@ typedef struct {
     PyObject *result;       /* most recently returned result tuple */
     Py_ssize_t r;           /* size of result tuple */
     int stopped;            /* set to 1 when the iterator is exhausted */
-} permutationsobject;
+};
 
 #define permutationsobject_CAST(op) ((permutationsobject *)(op))
 
@@ -2942,10 +2938,16 @@ permutations_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.permutations.__next__ as permutations_next
+[clinic start generated code]*/
+
 static PyObject *
-permutations_next_lock_held(PyObject *op)
+permutations_next_impl(permutationsobject *self)
+/*[clinic end generated code: output=17d8db4465bd3ee5 input=d4c1a1eed76a35e0]*/
 {
-    permutationsobject *po = permutationsobject_CAST(op);
+    permutationsobject *po = self;
     PyObject *elem;
     PyObject *oldelem;
     PyObject *pool = po->pool;
@@ -3033,16 +3035,6 @@ permutations_next_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-permutations_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = permutations_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 static PyMethodDef permuations_methods[] = {
     {"__sizeof__", permutations_sizeof, METH_NOARGS, sizeof_doc},
     {NULL,              NULL}   /* sentinel */
@@ -3072,14 +3064,14 @@ static PyType_Spec permutations_spec = {
 
 /* accumulate object ********************************************************/
 
-typedef struct {
+struct accumulateobject {
     PyObject_HEAD
     PyObject *total;
     PyObject *it;
     PyObject *binop;
     PyObject *initial;
     itertools_state *state;
-} accumulateobject;
+};
 
 #define accumulateobject_CAST(op)   ((accumulateobject *)(op))
 
@@ -3149,10 +3141,16 @@ accumulate_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.accumulate.__next__ as accumulate_next
+[clinic start generated code]*/
+
 static PyObject *
-accumulate_next_lock_held(PyObject *op)
+accumulate_next_impl(accumulateobject *self)
+/*[clinic end generated code: output=30d3f9b40248d8a6 input=758a144b7cfb1ef4]*/
 {
-    accumulateobject *lz = accumulateobject_CAST(op);
+    accumulateobject *lz = self;
     PyObject *val, *newtotal;
 
     if (lz->initial != Py_None) {
@@ -3182,16 +3180,6 @@ accumulate_next_lock_held(PyObject *op)
     return newtotal;
 }
 
-static PyObject *
-accumulate_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = accumulate_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 static PyType_Slot accumulate_slots[] = {
     {Py_tp_dealloc, accumulate_dealloc},
     {Py_tp_getattro, PyObject_GenericGetAttr},
@@ -3852,14 +3840,14 @@ static PyType_Spec repeat_spec = {
 
 /* ziplongest object *********************************************************/
 
-typedef struct {
+struct ziplongestobject {
     PyObject_HEAD
     Py_ssize_t tuplesize;
     Py_ssize_t numactive;
     PyObject *ittuple;                  /* tuple of iterators */
     PyObject *result;
     PyObject *fillvalue;
-} ziplongestobject;
+};
 
 #define ziplongestobject_CAST(op)   ((ziplongestobject *)(op))
 
@@ -3955,10 +3943,16 @@ zip_longest_traverse(PyObject *op, visitproc visit, void *arg)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+itertools.zip_longest.__next__ as zip_longest_next
+[clinic start generated code]*/
+
 static PyObject *
-zip_longest_next_lock_held(PyObject *op)
+zip_longest_next_impl(ziplongestobject *self)
+/*[clinic end generated code: output=11bc756cf896c576 input=5b1e3b52098852a9]*/
 {
-    ziplongestobject *lz = ziplongestobject_CAST(op);
+    ziplongestobject *lz = self;
     Py_ssize_t i;
     Py_ssize_t tuplesize = lz->tuplesize;
     PyObject *result = lz->result;
@@ -4027,16 +4021,6 @@ zip_longest_next_lock_held(PyObject *op)
     return result;
 }
 
-static PyObject *
-zip_longest_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = zip_longest_next_lock_held(op);
-    Py_END_CRITICAL_SECTION()
-    return result;
-}
-
 PyDoc_STRVAR(zip_longest_doc,
 "zip_longest(*iterables, fillvalue=None)\n\
 --\n\
diff --git a/Objects/clinic/odictobject.c.h b/Objects/clinic/odictobject.c.h
index 92129e6444810f..ebef5dad597c54 100644
--- a/Objects/clinic/odictobject.c.h
+++ b/Objects/clinic/odictobject.c.h
@@ -452,4 +452,19 @@ OrderedDict_move_to_end(PyObject *self, PyObject *const *args, Py_ssize_t nargs,
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=89f7e92de998f9a4 input=a9049054013a1b77]*/
+
+static PyObject *
+odictiter_iternext_impl(PyObject *self);
+
+static PyObject *
+odictiter_iternext(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = odictiter_iternext_impl(self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+/*[clinic end generated code: output=b729b3f1d40afcd9 input=a9049054013a1b77]*/
diff --git a/Objects/clinic/rangeobject.c.h b/Objects/clinic/rangeobject.c.h
index d9142eddde4b17..1cdc2e43b50dbc 100644
--- a/Objects/clinic/rangeobject.c.h
+++ b/Objects/clinic/rangeobject.c.h
@@ -147,4 +147,19 @@ longrange_iterator___setstate__(PyObject *r, PyObject *state)
 
     return return_value;
 }
-/*[clinic end generated code: output=719c0e4c81fe0f4a input=a9049054013a1b77]*/
+
+static PyObject *
+longrangeiter_next_impl(longrangeiterobject *self);
+
+static PyObject *
+longrangeiter_next(PyObject *self)
+{
+    PyObject *return_value = NULL;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    return_value = longrangeiter_next_impl((longrangeiterobject *)self);
+    Py_END_CRITICAL_SECTION();
+
+    return return_value;
+}
+/*[clinic end generated code: output=e31cc65008f9db4f input=a9049054013a1b77]*/
diff --git a/Objects/odictobject.c b/Objects/odictobject.c
index a860ade3b148e5..b9283ad6469d08 100644
--- a/Objects/odictobject.c
+++ b/Objects/odictobject.c
@@ -478,8 +478,9 @@ Potential Optimizations
 
 /*[clinic input]
 class OrderedDict "PyODictObject *" "&PyODict_Type"
+class odictiter "PyObject *" "&PyODictIter_Type"
 [clinic start generated code]*/
-/*[clinic end generated code: output=da39a3ee5e6b4b0d input=ca0641cf6143d4af]*/
+/*[clinic end generated code: output=da39a3ee5e6b4b0d input=ae5599eeb14d29ae]*/
 
 
 typedef struct _odictnode _ODictNode;
@@ -1804,10 +1805,16 @@ odictiter_nextkey(odictiterobject *di)
     return res;
 }
 
+/*[clinic input]
+@critical_section
+odictiter.__next__ as odictiter_iternext
+[clinic start generated code]*/
+
 static PyObject *
-odictiter_iternext_lock_held(PyObject *op)
+odictiter_iternext_impl(PyObject *self)
+/*[clinic end generated code: output=71aef8eec9e1a133 input=bc8bec6c85481b93]*/
 {
-    odictiterobject *di = (odictiterobject*)op;
+    odictiterobject *di = (odictiterobject*)self;
     PyObject *result, *value;
     PyObject *key = odictiter_nextkey(di);  /* new reference */
 
@@ -1862,16 +1869,6 @@ odictiter_iternext_lock_held(PyObject *op)
     return NULL;
 }
 
-static PyObject *
-odictiter_iternext(PyObject *op)
-{
-    PyObject *res;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    res = odictiter_iternext_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return res;
-}
-
 
 /* No need for tp_clear because odictiterobject is not mutable. */
 
diff --git a/Objects/rangeobject.c b/Objects/rangeobject.c
index 55b7f108730728..2279d4ce17d2f6 100644
--- a/Objects/rangeobject.c
+++ b/Objects/rangeobject.c
@@ -1158,10 +1158,16 @@ longrangeiter_dealloc(PyObject *op)
     PyObject_Free(r);
 }
 
+/*[clinic input]
+@critical_section
+longrange_iterator.__next__ as longrangeiter_next
+[clinic start generated code]*/
+
 static PyObject *
-longrangeiter_next_lock_held(PyObject *op)
+longrangeiter_next_impl(longrangeiterobject *self)
+/*[clinic end generated code: output=49bf9a9b89c07ba5 input=2c4ac537c23a92d3]*/
 {
-    longrangeiterobject *r = (longrangeiterobject*)op;
+    longrangeiterobject *r = self;
     if (PyObject_RichCompareBool(r->len, _PyLong_GetZero(), Py_GT) != 1)
         return NULL;
 
@@ -1180,16 +1186,6 @@ longrangeiter_next_lock_held(PyObject *op)
     return result;
 }
 
-static PyObject *
-longrangeiter_next(PyObject *op)
-{
-    PyObject *result;
-    Py_BEGIN_CRITICAL_SECTION(op);
-    result = longrangeiter_next_lock_held(op);
-    Py_END_CRITICAL_SECTION();
-    return result;
-}
-
 PyTypeObject PyLongRangeIter_Type = {
         PyVarObject_HEAD_INIT(&PyType_Type, 0)
         "longrange_iterator",                   /* tp_name */
diff --git a/Tools/clinic/libclinic/clanguage.py b/Tools/clinic/libclinic/clanguage.py
index 1f71c6cda81b81..f8ab2fd2515461 100644
--- a/Tools/clinic/libclinic/clanguage.py
+++ b/Tools/clinic/libclinic/clanguage.py
@@ -12,7 +12,7 @@
 from libclinic.function import (
     Module, Class, Property, Function, Parameter,
     group_to_variable_name,
-    METHOD_INIT, METHOD_NEW,
+    METHOD_INIT, METHOD_NEW, SLOT,
     ACCESSORS, SETTERS)
 from libclinic.converters import self_converter
 from libclinic.parse_args import (
@@ -383,6 +383,10 @@ def render_function(
         if ("METH_O" in templates['methoddef_define'] and
             '{parser_parameters}' in templates['parser_prototype']):
             data.declarations.pop(0)
+        # A slot function receives its arguments as parameters, see
+        # parse_slot(): there is nothing to declare or parse.
+        if f.kind is SLOT:
+            data.declarations.clear()
 
         full_name = f.full_name
         template_dict = {'full_name': full_name}
@@ -404,12 +408,17 @@ def render_function(
         for converter in converters:
             converter.set_template_dict(template_dict)
 
-        if f.kind not in SETTERS | {METHOD_INIT}:
+        if f.kind not in SETTERS | {METHOD_INIT, SLOT}:
             f.return_converter.render(f, data)
         template_dict['impl_return_type'] = f.return_converter.type
-        # tp_init returns int; every other parser returns an object.
-        template_dict['return_type'] = (
-            'int' if f.kind is METHOD_INIT else 'PyObject *')
+        # tp_init returns int, a slot function returns what its slot
+        # declares, every other parser returns an object.
+        if f.kind is SLOT:
+            assert f.slot is not None
+            template_dict['return_type'] = f.slot.return_type
+        else:
+            template_dict['return_type'] = (
+                'int' if f.kind is METHOD_INIT else 'PyObject *')
 
         template_dict['declarations'] = libclinic.format_escape("\n".join(data.declarations))
         template_dict['initializers'] = "\n\n".join(data.initializers)
diff --git a/Tools/clinic/libclinic/converter.py b/Tools/clinic/libclinic/converter.py
index 0caa8a8956f999..2f9f54f7e29df4 100644
--- a/Tools/clinic/libclinic/converter.py
+++ b/Tools/clinic/libclinic/converter.py
@@ -275,6 +275,11 @@ def __getattr__(self, attr):
     else:  # pragma: no cover
         pass
 
+    @classmethod
+    def dsl_name(cls) -> str:
+        """The name of the converter in the DSL, e.g. 'object'."""
+        return cls.__name__.removesuffix('_converter')
+
     def converter_init(self) -> None:
         pass
 
diff --git a/Tools/clinic/libclinic/converters.py b/Tools/clinic/libclinic/converters.py
index 8db4c5fe22e26d..c54dab302b6658 100644
--- a/Tools/clinic/libclinic/converters.py
+++ b/Tools/clinic/libclinic/converters.py
@@ -8,7 +8,7 @@
 from libclinic import fail, NullType, unspecified, NULL, c_bytes_repr, c_unichar_repr
 from libclinic.function import (
     Function, Parameter,
-    CALLABLE, STATIC_METHOD, CLASS_METHOD, METHOD_INIT, METHOD_NEW,
+    CALLABLE, STATIC_METHOD, CLASS_METHOD, METHOD_INIT, METHOD_NEW, SLOT,
     ACCESSORS)
 from libclinic.codegen import CRenderData, TemplateDict
 from libclinic.converter import (
@@ -1197,7 +1197,7 @@ def correct_name_for_self(
         f: Function,
         parser: bool = False
 ) -> tuple[str, str]:
-    if f.kind in {CALLABLE, METHOD_INIT} | ACCESSORS:
+    if f.kind in {CALLABLE, METHOD_INIT, SLOT} | ACCESSORS:
         if f.cls:
             return "PyObject *", "self"
         return "PyObject *", "module"
diff --git a/Tools/clinic/libclinic/dsl_parser.py b/Tools/clinic/libclinic/dsl_parser.py
index 514844af1d05b7..60730b8828d843 100644
--- a/Tools/clinic/libclinic/dsl_parser.py
+++ b/Tools/clinic/libclinic/dsl_parser.py
@@ -18,7 +18,7 @@
     Module, Class, Property, Function, Parameter,
     FunctionKind,
     CALLABLE, STATIC_METHOD, CLASS_METHOD, METHOD_INIT, METHOD_NEW,
-    GETTER, SETTER, SETTER_AND_DELETER,
+    GETTER, SETTER, SETTER_AND_DELETER, SLOT,
     ACCESSORS, SETTERS)
 from libclinic.converter import (
     converters, legacy_converters)
@@ -28,12 +28,15 @@
 from libclinic.return_converters import (
     CReturnConverter, return_converters,
     int_return_converter)
+from libclinic.slots import SlotSpec, SLOTS
 from libclinic.parser import create_parser_namespace
 if TYPE_CHECKING:
     from libclinic.block_parser import Block
     from libclinic.app import Clinic
 
 
+# Special methods which Argument Clinic cannot convert.  The type slots
+# which it supports are listed in libclinic.slots.
 unsupported_special_methods: set[str] = set("""
 
 __abs__
@@ -76,7 +79,6 @@
 __mod__
 __mul__
 __neg__
-__next__
 __or__
 __pos__
 __pow__
@@ -265,6 +267,7 @@ class DSLParser:
     critical_section: bool
     target_critical_section: list[str]
     disable_fastcall: bool
+    slot: SlotSpec | None
     # Line of the file which is being parsed.
     line_number: int | None
     from_version_re = re.compile(r'([*/]) +\[from +(.+)\]')
@@ -312,6 +315,7 @@ def reset(self) -> None:
         self.target_critical_section = []
         self.disable_fastcall = False
         self.vectorcall: bool = False
+        self.slot = None
         self.permit_long_summary = False
         self.permit_long_docstring_body = False
 
@@ -603,6 +607,9 @@ def parse_function_names(self, line: str) -> FunctionNames:
             fields = full_name.split(".")
             if fields[-1] == '__new__':
                 fields.pop()
+            elif fields[-1] in SLOTS:
+                # list.__repr__ -> list_repr
+                fields[-1] = fields[-1].strip('_')
             c_basename = "_".join(fields)
         if not libclinic.is_legal_py_identifier(full_name):
             fail(f"Illegal function name: {full_name!r}")
@@ -627,12 +634,23 @@ def normalize_function_kind(self, fullname: str) -> None:
             fail("'__new__' must be a class method!")
         if self.kind in ACCESSORS and not cls:
             fail("@getter and @setter must be methods")
+        slot = SLOTS.get(name)
+        if slot is not None:
+            if self.kind is not CALLABLE:
+                fail(f"{name!r} must be a normal method; got '{self.kind}'!")
+            if not cls:
+                fail(f"{name!r} must be a method!")
+            if self.coexist:
+                fail(f"@coexist cannot be applied to {name!r}")
 
         # Normalise self.kind.
         if name == '__new__':
             self.kind = METHOD_NEW
         elif name == '__init__':
             self.kind = METHOD_INIT
+        elif slot is not None:
+            self.kind = SLOT
+            self.slot = slot
 
         # Validate @vectorcall usage.
         if self.vectorcall:
@@ -653,6 +671,9 @@ def resolve_return_converter(
                 fail("@setter methods cannot define a return type")
             if self.kind is METHOD_INIT:
                 fail("__init__ methods cannot define a return type")
+            if self.kind is SLOT:
+                fail(f"{full_name!r} cannot define a return type; "
+                     "its type slot defines it")
             ast_input = f"def x() -> {forced_converter}: pass"
             try:
                 module_node = ast.parse(ast_input)
@@ -672,6 +693,9 @@ def resolve_return_converter(
 
         if self.kind in {METHOD_INIT} | SETTERS:
             return int_return_converter()
+        if self.kind is SLOT:
+            assert self.slot is not None
+            return return_converters[self.slot.return_converter]()
         return CReturnConverter()
 
     def parse_cloned_function(self, names: FunctionNames, existing: str) -> None:
@@ -703,6 +727,8 @@ def parse_cloned_function(self, names: FunctionNames, existing: str) -> None:
             "docstring_line_number": None,
             "line_number": self.line_number,
         }
+        if existing_function.kind is SLOT or self.slot is not None:
+            fail("Type slot methods cannot be cloned")
         if not (existing_function.kind is self.kind and
                 existing_function.coexist == self.coexist):
             # Allow __new__ or __init__ methods.
@@ -777,6 +803,7 @@ def state_modulename_name(self, line: str) -> None:
             forced_text_signature=self.forced_text_signature,
             line_number=self.line_number,
             vectorcall=self.vectorcall,
+            slot=self.slot,
         )
         self.add_function(func)
 
@@ -893,6 +920,10 @@ def state_parameters_start(self, line: str) -> None:
         assert self.function is not None
         if self.function.kind is GETTER:
             fail("@getter methods cannot define parameters")
+        if self.function.kind is SLOT:
+            assert self.function.slot is not None
+            if not self.function.slot.takes_parameters:
+                fail(f"{self.function.name!r} methods cannot define parameters")
 
         self.parameter_continuation = ''
         return self.next(self.state_parameter, line)
@@ -1498,6 +1529,12 @@ def format_docstring(self) -> str:
         assert self.function is not None
         f = self.function
         # For the following special cases, it does not make sense to render a docstring.
+        if f.kind is SLOT:
+            # A slot function has no docstring (there is no PyMethodDef).
+            if f.docstring:
+                fail(f"{f.name!r} methods cannot have a docstring",
+                     line_number=f.docstring_line_number)
+            return f.docstring
         if f.kind in {METHOD_INIT, METHOD_NEW} | ACCESSORS and not f.docstring:
             return f.docstring
 
@@ -1639,6 +1676,21 @@ def check_vectorcall_parameters(self, lineno: int) -> None:
                      f"parse_arg(); parameter {p.name!r} does not",
                      line_number=lineno)
 
+    def check_slot_parameters(self, func: Function) -> None:
+        """Check that the parameters of a slot method select a C slot."""
+        assert func.slot is not None
+        params = list(func.parameters.values())[1:]
+        for p in params:
+            if not p.is_positional_only():
+                fail(f"The parameters of {func.name!r} must be positional-only")
+            if p.default is not unspecified:
+                fail(f"The parameters of {func.name!r} cannot have a default value")
+        converters = [p.converter for p in params]
+        if func.slot.c_slot(converters) is None:
+            got = ', '.join(c.dsl_name() for c in converters)
+            fail(f"{func.name!r} must declare "
+                 f"{func.slot.describe_signatures()}; got ({got})")
+
     def do_post_block_processing_cleanup(self, lineno: int) -> None:
         """
         Called when processing the block is done.
@@ -1647,6 +1699,8 @@ def do_post_block_processing_cleanup(self, lineno: int) -> None:
             return
 
         func = self.function
+        if func.kind is SLOT:
+            self.check_slot_parameters(func)
         if func.kind in (SETTER, SETTER_AND_DELETER):
             # The new value is the only parameter of a setter.  The setter
             # of a deletable attribute is also called with NULL to delete it,
diff --git a/Tools/clinic/libclinic/function.py b/Tools/clinic/libclinic/function.py
index e5d7469bb732aa..5270f4520180d4 100644
--- a/Tools/clinic/libclinic/function.py
+++ b/Tools/clinic/libclinic/function.py
@@ -11,6 +11,7 @@
     from libclinic.converters import self_converter
     from libclinic.return_converters import CReturnConverter
     from libclinic.app import Clinic
+    from libclinic.slots import SlotSpec
 
 from libclinic import VersionTuple, unspecified
 
@@ -94,6 +95,8 @@ class FunctionKind(enum.Enum):
     GETTER          = enum.auto()
     SETTER          = enum.auto()
     SETTER_AND_DELETER  = enum.auto()
+    # A type slot ("tp slot"), see libclinic.slots.
+    SLOT            = enum.auto()
 
     @functools.cached_property
     def new_or_init(self) -> bool:
@@ -111,6 +114,7 @@ def __repr__(self) -> str:
 GETTER: Final = FunctionKind.GETTER
 SETTER: Final = FunctionKind.SETTER
 SETTER_AND_DELETER: Final = FunctionKind.SETTER_AND_DELETER
+SLOT: Final = FunctionKind.SLOT
 
 # The kinds which implement the setter of an entry of PyGetSetDef.
 SETTERS: Final = frozenset({SETTER, SETTER_AND_DELETER})
@@ -156,6 +160,8 @@ class Function:
     # Line on which the docstring starts (`None` if there is no docstring).
     docstring_line_number: int | None = None
     vectorcall: bool = False
+    # The type slot which the function implements (kind is SLOT).
+    slot: SlotSpec | None = None
 
     def __post_init__(self) -> None:
         self.parent = self.cls or self.module
@@ -224,7 +230,8 @@ def render_parameters(self) -> list[Parameter]:
 
     @property
     def methoddef_flags(self) -> str | None:
-        if self.kind.new_or_init:
+        if self.kind.new_or_init or self.kind is SLOT:
+            # Slot functions are not stored in a PyMethodDef.
             return None
         flags = []
         match self.kind:
diff --git a/Tools/clinic/libclinic/parse_args.py b/Tools/clinic/libclinic/parse_args.py
index a3ab39376320de..73414e897e2615 100644
--- a/Tools/clinic/libclinic/parse_args.py
+++ b/Tools/clinic/libclinic/parse_args.py
@@ -6,7 +6,7 @@
 from libclinic.function import (
     Function, Parameter, ParamTuple,
     count_required, group_to_variable_name, permute_optional_groups,
-    GETTER, SETTER, SETTER_AND_DELETER, METHOD_INIT,
+    GETTER, SETTER, SETTER_AND_DELETER, METHOD_INIT, SLOT,
     ACCESSORS, SETTERS)
 from libclinic.converter import CConverter
 from libclinic.converters import (
@@ -142,6 +142,14 @@ def declare_parser(
     static int
     {c_basename}({self_type}{self_name}, PyObject *arg, void *Py_UNUSED(context))
 """)
+PARSER_PROTOTYPE_SLOT: Final[str] = libclinic.normalize_snippet("""
+    static {return_type}
+    {c_basename}({self_type}{self_name})
+""")
+PARSER_PROTOTYPE_SLOT_ARGS: Final[str] = libclinic.normalize_snippet("""
+    static {return_type}
+    {c_basename}({self_type}{self_name}, {parser_parameters})
+""")
 METH_O_PROTOTYPE: Final[str] = libclinic.normalize_snippet("""
     static PyObject *
     {c_basename}({self_type}{self_name}, {parser_parameters})
@@ -393,8 +401,11 @@ def use_meth_o(self) -> bool:
                 and not self.is_new_or_init())
 
     def use_simple_return(self) -> bool:
+        if self.func.critical_section:
+            return False
+        # A slot function returns the result of the impl unchanged.
         return (self.func.return_converter.type == 'PyObject *'
-                and not self.func.critical_section)
+                or self.func.kind is SLOT)
 
     def use_pyobject_self(self) -> bool:
         return self.self_parameter_converter.use_pyobject_self(self.func)
@@ -407,6 +418,19 @@ def select_prototypes(self) -> None:
 
         if self.is_new_or_init() and not self.func.docstring:
             pass
+        elif self.func.kind is SLOT:
+            # A slot function is not stored in a PyMethodDef and has no
+            # docstring (the DSL parser rejects one).  It returns what its
+            # slot declares.
+            assert not self.func.docstring
+            assert self.func.slot is not None
+            self.methoddef_define = ''
+            return_type = self.func.slot.return_type
+            if not return_type.endswith('*'):
+                return_type += ' '
+            self.return_value_declaration = (
+                f"{return_type}{{parser_retval}} = "
+                f"{self.func.slot.error_value};")
         elif self.func.kind is GETTER:
             # Only an accessor compiled conditionally announces itself.
             self.methoddef_define = (GETTERDEF_PROTOTYPE_DEFINE
@@ -482,6 +506,18 @@ def render_setter_value(self) -> str:
             libclinic.normalize_snippet("}}", indent=4),
         ])
 
+    def parse_slot(self) -> None:
+        """Generate the slot function.
+
+        It does not parse anything: it forwards its arguments to the impl
+        and, with @critical_section, holds the lock while doing so.
+        """
+        if self.parameters:
+            self.parser_prototype = PARSER_PROTOTYPE_SLOT_ARGS
+        else:
+            self.parser_prototype = PARSER_PROTOTYPE_SLOT
+        self.finish_parser_body([])
+
     def parse_accessor(self) -> None:
         """Generate the code of a getter or a setter."""
         parser_code: list[str] = []
@@ -1744,7 +1780,9 @@ def parse_args(self, clang: CLanguage) -> dict[str, str]:
         # previous call to parser_body. this is used for an awful hack.
         self.parser_body_fields: tuple[str, ...] = ()
 
-        if self.func.kind in ACCESSORS:
+        if self.func.kind is SLOT:
+            self.parse_slot()
+        elif self.func.kind in ACCESSORS:
             self.parse_accessor()
         elif not self.parameters and not self.varpos and not self.var_keyword:
             self.parse_no_args()
diff --git a/Tools/clinic/libclinic/slots.py b/Tools/clinic/libclinic/slots.py
new file mode 100644
index 00000000000000..ee38b06c2ce4a8
--- /dev/null
+++ b/Tools/clinic/libclinic/slots.py
@@ -0,0 +1,59 @@
+"""Type slots ("tp slots") for which Argument Clinic generates the slot function.
+
+A slot function does not parse arguments: Argument Clinic generates a thin
+wrapper with the C signature of the slot, which forwards its arguments to
+the implementation function.  The wrapper is the place where
+``@critical_section`` acquires the lock.
+
+The parameters declared in the DSL only select the C signature; they must be
+positional-only, without a default value. When a special method maps to
+several slots (``__getitem__`` is ``sq_item`` or ``mp_subscript``),
+the converters of its parameters determine which one it implements.
+"""
+from __future__ import annotations
+import dataclasses as dc
+from typing import Final
+
+from libclinic.converter import CConverter
+
+
+@dc.dataclass(frozen=True, slots=True)
+class SlotSpec:
+    # The special method name, e.g. '__next__'.
+    name: str
+    # The C return type of the slot function.
+    return_type: str
+    # The name of the return converter which declares `return_type`.
+    return_converter: str
+    # Maps the converter classes of the parameters (excluding self) to
+    # the C slot.
+    signatures: dict[tuple[type[CConverter], ...], str]
+
+    @property
+    def error_value(self) -> str:
+        """The value returned by the slot function on error."""
+        return 'NULL' if self.return_type.endswith('*') else '-1'
+
+    @property
+    def takes_parameters(self) -> bool:
+        return any(self.signatures)
+
+    def c_slot(self, converters: list[CConverter]) -> str | None:
+        """Return the C slot for the parameter converters, or None."""
+        classes = tuple(type(c) for c in converters)
+        return self.signatures.get(classes)
+
+    def describe_signatures(self) -> str:
+        return " or ".join(
+            f"({', '.join(c.dsl_name() for c in sig) or 'no parameters'})"
+            f" for {slot}"
+            for sig, slot in self.signatures.items())
+
+
+# Maps the name of a special method to its slot.  A name listed here must
+# not be in dsl_parser.unsupported_special_methods.
+SLOTS: Final[dict[str, SlotSpec]] = {
+    '__next__': SlotSpec('__next__', 'PyObject *', 'object', {
+        (): 'tp_iternext',
+    }),
+}