diff --git a/Include/object.h b/Include/object.h
--- a/Include/object.h
+++ b/Include/object.h
@@ -492,8 +492,10 @@
 PyAPI_FUNC(unsigned int) PyType_ClearCache(void);
 PyAPI_FUNC(void) PyType_Modified(PyTypeObject *);
 
+#ifndef Py_LIMITED_API
 PyAPI_FUNC(PyObject *) _PyType_GetDocFromInternalDoc(const char *name, const char *internal_doc);
 PyAPI_FUNC(PyObject *) _PyType_GetTextSignatureFromInternalDoc(const char *name, const char *internal_doc);
+#endif
 
 /* Generic operations on objects */
 struct _Py_Identifier;
diff --git a/Modules/_cursesmodule.c b/Modules/_cursesmodule.c
--- a/Modules/_cursesmodule.c
+++ b/Modules/_cursesmodule.c
@@ -136,7 +136,7 @@
 
 /*[clinic input]
 module curses
-class curses.window
+class curses.window "PyCursesWindowObject *" "&PyCursesWindow_Type"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -605,10 +605,10 @@
     {"addch", (PyCFunction)curses_window_addch, METH_VARARGS, curses_window_addch__doc__},
 
 static PyObject *
-curses_window_addch_impl(PyObject *self, int group_left_1, int x, int y, PyObject *ch, int group_right_1, long attr);
+curses_window_addch_impl(PyCursesWindowObject *self, int group_left_1, int x, int y, PyObject *ch, int group_right_1, long attr);
 
 static PyObject *
-curses_window_addch(PyObject *self, PyObject *args)
+curses_window_addch(PyCursesWindowObject *self, PyObject *args)
 {
     PyObject *return_value = NULL;
     int group_left_1 = 0;
@@ -650,8 +650,8 @@
 }
 
 static PyObject *
-curses_window_addch_impl(PyObject *self, int group_left_1, int x, int y, PyObject *ch, int group_right_1, long attr)
-/*[clinic end generated code: checksum=f6eeada77a9ec085125f3a27e4a2095f2a4c50be]*/
+curses_window_addch_impl(PyCursesWindowObject *self, int group_left_1, int x, int y, PyObject *ch, int group_right_1, long attr)
+/*[clinic end generated code: checksum=e1cdbd4f4e42fc6b36fd4755d7e4bd5b58751ea1]*/
 {
     PyCursesWindowObject *cwself = (PyCursesWindowObject *)self;
     int coordinates_group = group_left_1;
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -18,7 +18,7 @@
 
 /*[clinic input]
 module datetime
-class datetime.datetime
+class datetime.datetime "PyDateTime_DateTime *" "&PyDateTime_DateTimeType"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -4171,10 +4171,10 @@
     {"now", (PyCFunction)datetime_datetime_now, METH_VARARGS|METH_KEYWORDS|METH_CLASS, datetime_datetime_now__doc__},
 
 static PyObject *
-datetime_datetime_now_impl(PyTypeObject *type, PyObject *tz);
+datetime_datetime_now_impl(PyDateTime_DateTime *type, PyObject *tz);
 
 static PyObject *
-datetime_datetime_now(PyTypeObject *type, PyObject *args, PyObject *kwargs)
+datetime_datetime_now(PyDateTime_DateTime *type, PyObject *args, PyObject *kwargs)
 {
     PyObject *return_value = NULL;
     static char *_keywords[] = {"tz", NULL};
@@ -4191,8 +4191,8 @@
 }
 
 static PyObject *
-datetime_datetime_now_impl(PyTypeObject *type, PyObject *tz)
-/*[clinic end generated code: checksum=a6d3ad2c0ab6389075289af3467f7b8eb13f5f5c]*/
+datetime_datetime_now_impl(PyDateTime_DateTime *type, PyObject *tz)
+/*[clinic end generated code: checksum=a08bda09f461ab139673f24a022c728b9b2c6e0e]*/
 {
     PyObject *self;
 
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -30,7 +30,7 @@
 
 /*[clinic input]
 module dbm
-class dbm.dbm
+class dbm.dbm "dbmobject *" "&Dbmtype"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -6,28 +6,13 @@
 
 /*[clinic input]
 module _pickle
-class _pickle.Pickler
-class _pickle.PicklerMemoProxy
-class _pickle.Unpickler
-class _pickle.UnpicklerMemoProxy
+class _pickle.Pickler "PicklerObject *" "&Pickler_Type"
+class _pickle.PicklerMemoProxy "PicklerMemoProxyObject *" "&PicklerMemoProxyType"
+class _pickle.Unpickler "UnpicklerObject *" "&Unpickler_Type"
+class _pickle.UnpicklerMemoProxy "UnpicklerMemoProxyObject *" "&UnpicklerMemoProxyType"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
-/*[python input]
-class PicklerObject_converter(self_converter):
-    type = "PicklerObject *"
-
-class PicklerMemoProxyObject_converter(self_converter):
-    type = "PicklerMemoProxyObject *"
-
-class UnpicklerObject_converter(self_converter):
-    type = "UnpicklerObject *"
-
-class UnpicklerMemoProxyObject_converter(self_converter):
-    type = "UnpicklerMemoProxyObject *"
-[python start generated code]*/
-/*[python end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
-
 /* Bump this when new opcodes are added to the pickle protocol. */
 enum {
     HIGHEST_PROTOCOL = 4,
@@ -3878,8 +3863,6 @@
 
 _pickle.Pickler.clear_memo
 
-  self: PicklerObject
-
 Clears the pickler's "memo".
 
 The memo is the data structure that remembers which objects the
@@ -3923,7 +3906,6 @@
 
 _pickle.Pickler.dump
 
-  self: PicklerObject
   obj: object
   /
 
@@ -4018,7 +4000,6 @@
 
 _pickle.Pickler.__init__
 
-  self: PicklerObject
   file: object
   protocol: object = NULL
   fix_imports: bool = True
@@ -4158,8 +4139,6 @@
 /*[clinic input]
 _pickle.PicklerMemoProxy.clear
 
-  self: PicklerMemoProxyObject
-
 Remove all items from memo.
 [clinic start generated code]*/
 
@@ -4191,8 +4170,6 @@
 /*[clinic input]
 _pickle.PicklerMemoProxy.copy
 
-  self: PicklerMemoProxyObject
-
 Copy the memo to a new object.
 [clinic start generated code]*/
 
@@ -4254,8 +4231,6 @@
 /*[clinic input]
 _pickle.PicklerMemoProxy.__reduce__
 
-  self: PicklerMemoProxyObject
-
 Implement pickle support.
 [clinic start generated code]*/
 
@@ -6310,17 +6285,17 @@
     {"load", (PyCFunction)_pickle_Unpickler_load, METH_NOARGS, _pickle_Unpickler_load__doc__},
 
 static PyObject *
-_pickle_Unpickler_load_impl(PyObject *self);
+_pickle_Unpickler_load_impl(UnpicklerObject *self);
 
 static PyObject *
-_pickle_Unpickler_load(PyObject *self, PyObject *Py_UNUSED(ignored))
+_pickle_Unpickler_load(UnpicklerObject *self, PyObject *Py_UNUSED(ignored))
 {
     return _pickle_Unpickler_load_impl(self);
 }
 
 static PyObject *
-_pickle_Unpickler_load_impl(PyObject *self)
-/*[clinic end generated code: checksum=fb1119422c5e03045d690d1cd6c457f1ca4c585d]*/
+_pickle_Unpickler_load_impl(UnpicklerObject *self)
+/*[clinic end generated code: checksum=5ccece694e9898856d916e0a87f0133d4537ebb9]*/
 {
     UnpicklerObject *unpickler = (UnpicklerObject*)self;
 
@@ -6347,7 +6322,6 @@
 
 _pickle.Unpickler.find_class
 
-  self: UnpicklerObject
   module_name: object
   global_name: object
   /
@@ -6551,7 +6525,6 @@
 
 _pickle.Unpickler.__init__
 
-  self: UnpicklerObject
   file: object
   *
   fix_imports: bool = True
@@ -6692,8 +6665,6 @@
 /*[clinic input]
 _pickle.UnpicklerMemoProxy.clear
 
-  self: UnpicklerMemoProxyObject
-
 Remove all items from memo.
 [clinic start generated code]*/
 
@@ -6727,8 +6698,6 @@
 /*[clinic input]
 _pickle.UnpicklerMemoProxy.copy
 
-  self: UnpicklerMemoProxyObject
-
 Copy the memo to a new object.
 [clinic start generated code]*/
 
@@ -6783,8 +6752,6 @@
 /*[clinic input]
 _pickle.UnpicklerMemoProxy.__reduce__
 
-  self: UnpicklerMemoProxyObject
-
 Implement pickling support.
 [clinic start generated code]*/
 
diff --git a/Modules/_sre.c b/Modules/_sre.c
--- a/Modules/_sre.c
+++ b/Modules/_sre.c
@@ -528,11 +528,10 @@
 
 /*[clinic input]
 module _sre
-class _sre.SRE_Pattern
+class _sre.SRE_Pattern "PatternObject *" "&Pattern_Type"
 
 _sre.SRE_Pattern.match as pattern_match
 
-    self: self(type="PatternObject *")
     pattern: object
     pos: Py_ssize_t = 0
     endpos: Py_ssize_t(c_default="PY_SSIZE_T_MAX") = sys.maxsize
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -19,7 +19,7 @@
 
 /*[clinic input]
 module unicodedata
-class unicodedata.UCD
+class unicodedata.UCD 'PreviousDBVersion *' '&UCD_Type'
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -140,10 +140,10 @@
     {"decimal", (PyCFunction)unicodedata_UCD_decimal, METH_VARARGS, unicodedata_UCD_decimal__doc__},
 
 static PyObject *
-unicodedata_UCD_decimal_impl(PyObject *self, PyUnicodeObject *unichr, PyObject *default_value);
+unicodedata_UCD_decimal_impl(PreviousDBVersion *self, PyUnicodeObject *unichr, PyObject *default_value);
 
 static PyObject *
-unicodedata_UCD_decimal(PyObject *self, PyObject *args)
+unicodedata_UCD_decimal(PreviousDBVersion *self, PyObject *args)
 {
     PyObject *return_value = NULL;
     PyUnicodeObject *unichr;
@@ -160,8 +160,8 @@
 }
 
 static PyObject *
-unicodedata_UCD_decimal_impl(PyObject *self, PyUnicodeObject *unichr, PyObject *default_value)
-/*[clinic end generated code: checksum=01826b179d497d8fd3842c56679ecbd4faddaa95]*/
+unicodedata_UCD_decimal_impl(PreviousDBVersion *self, PyUnicodeObject *unichr, PyObject *default_value)
+/*[clinic end generated code: checksum=e1371a1a016e19fdd3cd2c1af1d1832df095f50b]*/
 {
     PyUnicodeObject *v = (PyUnicodeObject *)unichr;
     int have_old = 0;
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -83,8 +83,8 @@
 
 /*[clinic input]
 module zlib
-class zlib.Compress
-class zlib.Decompress
+class zlib.Compress "compobject *" "&Comptype"
+class zlib.Decompress "compobject *" "&Decomptype"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -748,8 +748,6 @@
 
 zlib.Decompress.decompress
 
-    self: self(type="compobject *")
-
     data: Py_buffer
         The binary data to decompress.
     max_length: uint = 0
@@ -1030,8 +1028,6 @@
 /*[clinic input]
 zlib.Compress.copy
 
-    self: self(type="compobject *")
-
 Return a copy of the compression object.
 [clinic start generated code]*/
 
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -70,7 +70,7 @@
 #include "stringlib/eq.h"
 
 /*[clinic input]
-class dict
+class dict "PyDictObject *" "&PyDict_Type"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -1710,10 +1710,10 @@
     {"fromkeys", (PyCFunction)dict_fromkeys, METH_VARARGS|METH_CLASS, dict_fromkeys__doc__},
 
 static PyObject *
-dict_fromkeys_impl(PyTypeObject *type, PyObject *iterable, PyObject *value);
+dict_fromkeys_impl(PyDictObject *type, PyObject *iterable, PyObject *value);
 
 static PyObject *
-dict_fromkeys(PyTypeObject *type, PyObject *args)
+dict_fromkeys(PyDictObject *type, PyObject *args)
 {
     PyObject *return_value = NULL;
     PyObject *iterable;
@@ -1730,8 +1730,8 @@
 }
 
 static PyObject *
-dict_fromkeys_impl(PyTypeObject *type, PyObject *iterable, PyObject *value)
-/*[clinic end generated code: checksum=008269e1774a379b356841548c04061fd78a9542]*/
+dict_fromkeys_impl(PyDictObject *type, PyObject *iterable, PyObject *value)
+/*[clinic end generated code: checksum=86c2e7b5324db1321b80c02092a4e5bce10a5777]*/
 {
     PyObject *it;       /* iter(seq) */
     PyObject *key;
@@ -2217,8 +2217,8 @@
     {"__contains__", (PyCFunction)dict___contains__, METH_O|METH_COEXIST, dict___contains____doc__},
 
 static PyObject *
-dict___contains__(PyObject *self, PyObject *key)
-/*[clinic end generated code: checksum=c4f85a39baac4776c4275ad5f072f7732c5f0806]*/
+dict___contains__(PyDictObject *self, PyObject *key)
+/*[clinic end generated code: checksum=744ca54369dda9815a596304087f1b37fafa5960]*/
 {
     register PyDictObject *mp = (PyDictObject *)self;
     Py_hash_t hash;
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -48,7 +48,7 @@
 #endif
 
 /*[clinic input]
-class str
+class str "PyUnicodeObject *" "&PyUnicode_Type"
 [clinic start generated code]*/
 /*[clinic end generated code: checksum=da39a3ee5e6b4b0d3255bfef95601890afd80709]*/
 
@@ -12903,7 +12903,7 @@
 unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z);
 
 static PyObject *
-unicode_maketrans(void *null, PyObject *args)
+unicode_maketrans(PyUnicodeObject *null, PyObject *args)
 {
     PyObject *return_value = NULL;
     PyObject *x;
@@ -12922,7 +12922,7 @@
 
 static PyObject *
 unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
-/*[clinic end generated code: checksum=90a3de8c494b304687e1e0d7e5fa8ba78eac6533]*/
+/*[clinic end generated code: checksum=5d41682159e78e404fd34489dc2ddea33045eaa7]*/
 {
     PyObject *new = NULL, *key, *value;
     Py_ssize_t i = 0;
diff --git a/Tools/clinic/clinic.py b/Tools/clinic/clinic.py
--- a/Tools/clinic/clinic.py
+++ b/Tools/clinic/clinic.py
@@ -299,6 +299,10 @@
         # Should be full lines with \n eol characters.
         self.initializers = []
 
+        # The C statements needed to dynamically modify the values
+        # parsed by the parse call, before calling the impl.
+        self.modifications = []
+
         # The entries for the "keywords" array for PyArg_ParseTuple.
         # Should be individual strings representing the names.
         self.keywords = []
@@ -540,6 +544,7 @@
 
 parser_definition_impl_call
 
+    {modifications}
     {return_value} = {c_basename}_impl({impl_arguments});
 __________________________________________________
 
@@ -574,14 +579,14 @@
 
 parser_definition_no_positional
 
-    if (!_PyArg_NoPositional("{name}", args))
+    if ({self_type_check}!_PyArg_NoPositional("{name}", args))
         goto exit;
 
 __________________________________________________
 
 parser_definition_no_keywords
 
-    if (!_PyArg_NoKeywords("{name}", kwargs))
+    if ({self_type_check}!_PyArg_NoKeywords("{name}", kwargs))
         goto exit;
 
 __________________________________________________
@@ -690,7 +695,7 @@
         impl_definition = templates['impl_definition']
         impl_prototype = parser_prototype = parser_definition = None
 
-        parser_body_fields = None
+        parser_body_fields = ()
         def parser_body(prototype, *fields):
             nonlocal parser_body_fields
             add, output = text_accumulator()
@@ -1024,6 +1029,7 @@
 
         template_dict['docstring'] = self.docstring_for_c_string(f)
 
+        template_dict['self_name'] = template_dict['self_type'] = template_dict['self_type_check'] = ''
         f_self.converter.set_template_dict(template_dict)
 
         f.return_converter.render(f, data)
@@ -1031,6 +1037,7 @@
 
         template_dict['declarations'] = "\n".join(data.declarations)
         template_dict['initializers'] = "\n\n".join(data.initializers)
+        template_dict['modifications'] = '\n\n'.join(data.modifications)
         template_dict['keywords'] = '"' + '", "'.join(data.keywords) + '"'
         template_dict['format_units'] = ''.join(data.format_units)
         template_dict['parse_arguments'] = ', '.join(data.parse_arguments)
@@ -1059,6 +1066,7 @@
                 declarations=template_dict['declarations'],
                 return_conversion=template_dict['return_conversion'],
                 initializers=template_dict['initializers'],
+                modifications=template_dict['modifications'],
                 cleanup=template_dict['cleanup'],
                 )
 
@@ -1475,7 +1483,7 @@
         self.add_destination("buffer", "buffer")
         self.add_destination("two-pass", "two-pass")
         if filename:
-            self.add_destination("file", "file", "{basename}.clinic{extension}")
+            self.add_destination("file", "file", "clinic/{filename}.h")
 
         d = self.destinations.get
         self.field_destinations = collections.OrderedDict((
@@ -1571,6 +1579,12 @@
 
                 if destination.type == 'file':
                     try:
+                        dirname = os.path.basename(destination.filename)
+                        try:
+                            os.makedirs(dirname)
+                        except FileExistsError:
+                            if not os.path.isdir(dirname):
+                                fail("Can't write to destination " + repr(destination.filename) + ", can't make directory " + repr(dirname))
                         with open(destination.filename, "rt") as f:
                             parser_2 = BlockParser(f.read(), language=self.language)
                             blocks = list(parser_2)
@@ -1695,10 +1709,12 @@
         return "<clinic.Module " + repr(self.name) + " at " + str(id(self)) + ">"
 
 class Class:
-    def __init__(self, name, module=None, cls=None):
+    def __init__(self, name, module=None, cls=None, typedef=None, type_object=None):
         self.name = name
         self.module = module
         self.cls = cls
+        self.typedef = typedef
+        self.type_object = type_object
         self.parent = cls or module
 
         self.classes = collections.OrderedDict()
@@ -1979,6 +1995,7 @@
 
     # Should we show this parameter in the generated
     # __text_signature__? This is *almost* always True.
+    # (It's only False for __new__, __init__, and METH_STATIC functions.)
     show_in_signature = True
 
     # Overrides the name used in a text signature.
@@ -2049,6 +2066,11 @@
         if initializers:
             data.initializers.append('/* initializers for ' + name + ' */\n' + initializers.rstrip())
 
+        # modifications
+        modifications = self.modify()
+        if modifications:
+            data.modifications.append('/* modifications for ' + name + ' */\n' + modifications.rstrip())
+
         # keywords
         data.keywords.append(original_name)
 
@@ -2151,6 +2173,14 @@
         """
         return ""
 
+    def modify(self):
+        """
+        The C statements required to modify this variable after parsing.
+        Returns a string containing this code indented at column 0.
+        If no initialization is necessary, returns an empty string.
+        """
+        return ""
+
     def cleanup(self):
         """
         The C statements required to clean up after this variable.
@@ -2553,6 +2583,7 @@
     def set_template_dict(self, template_dict):
         template_dict['self_name'] = self.name
         template_dict['self_type'] = self.parser_type
+        template_dict['self_type_check'] = '({self_name} == {self_type_object}) &&\n        '
 
 
 
@@ -2807,6 +2838,7 @@
         self.keyword_only = False
         self.group = 0
         self.parameter_state = self.ps_start
+        self.seen_positional_with_default = False
         self.indent = IndentStack()
         self.kind = CALLABLE
         self.coexist = False
@@ -2824,11 +2856,15 @@
         module, cls = self.clinic._module_and_class(fields)
         if cls:
             fail("Can't nest a module inside a class!")
+
+        if name in module.classes:
+            fail("Already defined module " + repr(name) + "!")
+
         m = Module(name, module)
         module.modules[name] = m
         self.block.signatures.append(m)
 
-    def directive_class(self, name):
+    def directive_class(self, name, typedef, type_object):
         fields = name.split('.')
         in_classes = False
         parent = self
@@ -2836,11 +2872,12 @@
         so_far = []
         module, cls = self.clinic._module_and_class(fields)
 
-        c = Class(name, module, cls)
-        if cls:
-            cls.classes[name] = c
-        else:
-            module.classes[name] = c
+        parent = cls or module
+        if name in parent.classes:
+            fail("Already defined class " + repr(name) + "!")
+
+        c = Class(name, module, cls, typedef, type_object)
+        parent.classes[name] = c
         self.block.signatures.append(c)
 
     def directive_set(self, name, value):
@@ -3034,6 +3071,8 @@
                 else:
                     existing_function = None
                 if not existing_function:
+                    print("class", cls, "module", module, "exsiting", existing)
+                    print("cls. functions", cls.functions)
                     fail("Couldn't find existing function " + repr(existing) + "!")
 
                 fields = [x.strip() for x in full_name.split('.')]
@@ -3113,7 +3152,10 @@
 
         # insert a self converter automatically
         _, name = correct_name_for_self(self.function)
-        sc = self.function.self_converter = self_converter(name, self.function)
+        kwargs = {}
+        if cls:
+            kwargs['type'] = cls.typedef
+        sc = self.function.self_converter = self_converter(name, self.function, **kwargs)
         p_self = Parameter(sc.name, inspect.Parameter.POSITIONAL_ONLY, function=self.function, converter=sc)
         self.function.parameters[sc.name] = p_self
 
@@ -3174,18 +3216,21 @@
     # "parameter_state".  (Previously the code was a miasma of ifs and
     # separate boolean state variables.)  The states are:
     #
-    #  [ [ a, b, ] c, ] d, e, f, [ g, h, [ i ] ] /   <- line
-    # 01   2          3          4           5   6   <- state transitions
+    #  [ [ a, b, ] c, ] d, e, f=3, [ g, h, [ i ] ] /   <- line
+    # 01   2          3       4    5           6   7   <- state transitions
     #
     # 0: ps_start.  before we've seen anything.  legal transitions are to 1 or 3.
     # 1: ps_left_square_before.  left square brackets before required parameters.
     # 2: ps_group_before.  in a group, before required parameters.
-    # 3: ps_required.  required parameters.  (renumber left groups!)
-    # 4: ps_group_after.  in a group, after required parameters.
-    # 5: ps_right_square_after.  right square brackets after required parameters.
-    # 6: ps_seen_slash.  seen slash.
+    # 3: ps_required.  required parameters, positional-or-keyword or positional-only
+    #     (we don't know yet).  (renumber left groups!)
+    # 4: ps_optional.  positional-or-keyword or positional-only parameters that
+    #    now must have default values.
+    # 5: ps_group_after.  in a group, after required parameters.
+    # 6: ps_right_square_after.  right square brackets after required parameters.
+    # 7: ps_seen_slash.  seen slash.
     ps_start, ps_left_square_before, ps_group_before, ps_required, \
-    ps_group_after, ps_right_square_after, ps_seen_slash = range(7)
+    ps_optional, ps_group_after, ps_right_square_after, ps_seen_slash = range(8)
 
     def state_parameters_start(self, line):
         if self.ignore_line(line):
@@ -3244,21 +3289,25 @@
         elif self.parameter_state == self.ps_group_before:
             if not self.group:
                 self.to_required()
-        elif self.parameter_state == self.ps_group_after:
+        elif self.parameter_state in (self.ps_group_after, self.ps_optional):
             pass
         else:
-            fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ")")
+            fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ".a)")
 
         base, equals, default = line.rpartition('=')
         if not equals:
             base = default
             default = None
+
         module = None
         try:
             ast_input = "def x({}): pass".format(base)
             module = ast.parse(ast_input)
         except SyntaxError:
             try:
+                # the last = was probably inside a function call, like
+                #   i: int(nullable=True)
+                # so assume there was no actual default value.
                 default = None
                 ast_input = "def x({}): pass".format(line)
                 module = ast.parse(ast_input)
@@ -3274,16 +3323,20 @@
         name, legacy, kwargs = self.parse_converter(parameter.annotation)
 
         if not default:
+            if self.parameter_state == self.ps_optional:
+                fail("Can't have a parameter without a default (" + repr(parameter_name) + ")\nafter a parameter with a default!")
             value = unspecified
             if 'py_default' in kwargs:
                 fail("You can't specify py_default without specifying a default value!")
         else:
+            if self.parameter_state == self.ps_required:
+                self.parameter_state = self.ps_optional
             default = default.strip()
             ast_input = "x = {}".format(default)
+            bad = False
             try:
                 module = ast.parse(ast_input)
 
-                bad = False
                 if 'c_default' not in kwargs:
                     # we can only represent very simple data values in C.
                     # detect whether default is okay, via a blacklist
@@ -3316,9 +3369,19 @@
                     blacklist.visit(module)
                     bad = blacklist.bad
                 else:
-                    # if they specify a c_default, we can be more lenient about the default value.
-                    # but at least ensure that we can turn it into text and reconstitute it correctly.
-                    bad = default != repr(eval(default))
+                    # if they specify a c_default, we're way more lenient about
+                    # the default value.  as long as the string evals, and it's
+                    # not "unspecified", we'll assume it's fine.
+                    try:
+                        value = eval(default)
+                        if value == unspecified:
+                            fail("'unspecified' is not a legal default value!")
+                    except NameError:
+                        pass # probably an unrecognized name constant, that's okay
+                    except Exception as e:
+                        fail("Malformed expression given as default value\n"
+                             "{!r} raised {!r}".format(default, e))
+
                 if bad:
                     fail("Unsupported expression as default value: " + repr(default))
 
@@ -3432,7 +3495,7 @@
             elif self.parameter_state in (self.ps_required, self.ps_group_after):
                 self.parameter_state = self.ps_group_after
             else:
-                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ")")
+                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ".b)")
             self.group += 1
         elif symbol == ']':
             if not self.group:
@@ -3445,12 +3508,12 @@
             elif self.parameter_state in (self.ps_group_after, self.ps_right_square_after):
                 self.parameter_state = self.ps_right_square_after
             else:
-                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ")")
+                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ".c)")
         elif symbol == '/':
-            # ps_required is allowed here, that allows positional-only without option groups
+            # ps_required and ps_optional are allowed here, that allows positional-only without option groups
             # to work (and have default values!)
-            if (self.parameter_state not in (self.ps_required, self.ps_right_square_after, self.ps_group_before)) or self.group:
-                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ")")
+            if (self.parameter_state not in (self.ps_required, self.ps_optional, self.ps_right_square_after, self.ps_group_before)) or self.group:
+                fail("Function " + self.function.name + " has an unsupported group configuration. (Unexpected state " + str(self.parameter_state) + ".d)")
             if self.keyword_only:
                 fail("Function " + self.function.name + " mixes keyword-only and positional-only parameters, which is unsupported.")
             self.parameter_state = self.ps_seen_slash