gh-64490: Fix bugs in argument clinic varargs processing

Lib/test/clinic.test

@@ -3379,7 +3379,7 @@ test_vararg(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject
     static const char * const _keywords[] = {"a", NULL};
     static _PyArg_Parser _parser = {NULL, _keywords, "test_vararg", 0};
     PyObject *argsbuf[2];
-    Py_ssize_t noptargs = 0 + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 1;
+    Py_ssize_t noptargs = Py_MIN(nargs, 1) + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 1;
     PyObject *a;
     PyObject *__clinic_args = NULL;
 
@@ -3398,7 +3398,7 @@ exit:
 
 static PyObject *
 test_vararg_impl(PyObject *module, PyObject *a, PyObject *args)
-/*[clinic end generated code: output=f721025731c3bfe8 input=81d33815ad1bae6e]*/
+/*[clinic end generated code: output=f0b24e19c9dc129b input=81d33815ad1bae6e]*/
 
 /*[clinic input]
 test_vararg_with_default
@@ -3429,7 +3429,7 @@ test_vararg_with_default(PyObject *module, PyObject *const *args, Py_ssize_t nar
     static const char * const _keywords[] = {"a", "b", NULL};
     static _PyArg_Parser _parser = {NULL, _keywords, "test_vararg_with_default", 0};
     PyObject *argsbuf[3];
-    Py_ssize_t noptargs = 0 + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 1;
+    Py_ssize_t noptargs = Py_MIN(nargs, 1) + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 1;
     PyObject *a;
     PyObject *__clinic_args = NULL;
     int b = 0;
@@ -3458,7 +3458,7 @@ exit:
 static PyObject *
 test_vararg_with_default_impl(PyObject *module, PyObject *a, PyObject *args,
                               int b)
-/*[clinic end generated code: output=63b34d3241c52fda input=6e110b54acd9b22d]*/
+/*[clinic end generated code: output=0065e7b11eeb4826 input=6e110b54acd9b22d]*/
 
 /*[clinic input]
 test_vararg_with_only_defaults

Python/getargs.c

@@ -2571,7 +2571,7 @@ _PyArg_UnpackKeywordsWithVararg(PyObject *const *args, Py_ssize_t nargs,
             current_arg = NULL;
         }
 
-        buf[i + vararg + 1] = current_arg;
+        buf[i + 1] = current_arg;
 
         if (current_arg) {
             --nkwargs;

Tools/clinic/clinic.py

@@ -650,7 +650,7 @@ def output_templates(self, f):
         vararg = NO_VARARG
         pos_only = min_pos = max_pos = min_kw_only = pseudo_args = 0
         for i, p in enumerate(parameters, 1):
-            if p.is_keyword_only() or vararg != NO_VARARG:
+            if p.is_keyword_only():
                 assert not p.is_positional_only()
                 if not p.is_optional():
                     min_kw_only = i - max_pos
@@ -941,13 +941,15 @@ def parser_body(prototype, *fields, declarations=''):
                     max_pos,
                     min_kw_only
                 )
+                pre_buffer = "nargs"
             else:
                 args_declaration = "_PyArg_UnpackKeywordsWithVararg", "%s, %s, %s, %s" % (
                     min_pos,
                     max_pos,
                     min_kw_only,
                     vararg
                 )
+                pre_buffer = f"Py_MIN(nargs, {max_pos})" if max_pos else "0"
             if not new_or_init:
                 flags = "METH_FASTCALL|METH_KEYWORDS"
                 parser_prototype = parser_prototype_fastcall_keywords
@@ -958,7 +960,6 @@ def parser_body(prototype, *fields, declarations=''):
                     PyObject *argsbuf[%s];
                     """ % len(converters))
                 if has_optional_kw:
-                    pre_buffer = "0" if vararg != NO_VARARG else "nargs"
                     declarations += "\nPy_ssize_t noptargs = %s + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - %d;" % (pre_buffer, min_pos + min_kw_only)
                 parser_code = [normalize_snippet("""
                     args = %s(args, nargs, NULL, kwnames, &_parser, %s, argsbuf);
@@ -979,7 +980,7 @@ def parser_body(prototype, *fields, declarations=''):
                     Py_ssize_t nargs = PyTuple_GET_SIZE(args);
                     """ % len(converters))
                 if has_optional_kw:
-                    declarations += "\nPy_ssize_t noptargs = nargs + (kwargs ? PyDict_GET_SIZE(kwargs) : 0) - %d;" % (min_pos + min_kw_only)
+                    declarations += "\nPy_ssize_t noptargs = %s + (kwargs ? PyDict_GET_SIZE(kwargs) : 0) - %d;" % (pre_buffer, min_pos + min_kw_only)
                 parser_code = [normalize_snippet("""
                     fastargs = %s(_PyTuple_CAST(args)->ob_item, nargs, kwargs, NULL, &_parser, %s, argsbuf);
                     if (!fastargs) {{