gh-86199: Fix CI: move kwargs conversion out of the instruction body

Taking the address of a local (&dupkey) inside _MAKE_CALLARGS_A_TUPLE
prevents MSVC from emitting the required tail call in the tail-calling
interpreter build (error C4737). Move the mapping-to-dict conversion into
a new helper, _PyEval_KwargsToDict(), so the op body has no address-taken
locals.

Also update test_dict_merge: f(**d) no longer emits DICT_MERGE, so use
f(**d, **e) there and add a test that f(**d) traces through
_MAKE_CALLARGS_A_TUPLE without _DICT_MERGE.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: eendebakpt

Include/internal/pycore_ceval.h

@@ -298,6 +298,7 @@ PyAPI_FUNC(void) _PyEval_FormatAwaitableError(PyThreadState *tstate, PyTypeObjec
 PyAPI_FUNC(void) _PyEval_FormatExcCheckArg(PyThreadState *tstate, PyObject *exc, const char *format_str, PyObject *obj);
 PyAPI_FUNC(void) _PyEval_FormatExcUnbound(PyThreadState *tstate, PyCodeObject *co, int oparg);
 PyAPI_FUNC(void) _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwargs, PyObject *dupkey);
+PyAPI_FUNC(PyObject *) _PyEval_KwargsToDict(PyThreadState *tstate, PyObject *func, PyObject *kwargs);
 PyAPI_FUNC(PyObject *) _PyEval_ImportFrom(PyThreadState *, PyObject *, PyObject *);
 
 PyAPI_FUNC(PyObject *) _PyEval_LazyImportName(

Lib/test/test_capi/test_opt.py

@@ -5800,11 +5800,12 @@ def testfunc(n):
 
     def test_dict_merge(self):
         def testfunc(n):
-            d = {"a": 1, "b": 2}
+            d = {"a": 1}
+            e = {"b": 2}
             def f(**kwargs):
                 return kwargs
             for _ in range(n):
-                x = f(**d)
+                x = f(**d, **e)
             return x
 
         res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
@@ -5815,6 +5816,23 @@ def f(**kwargs):
         self.assertGreaterEqual(count_ops(ex, "_POP_TOP_NOP"), 1)
         self.assertLessEqual(count_ops(ex, "_POP_TOP"), 2)
 
+    def test_call_function_ex_single_kwargs_unpack(self):
+        # gh-86199: f(**d) no longer copies d with DICT_MERGE
+        def testfunc(n):
+            d = {"a": 1, "b": 2}
+            def f(**kwargs):
+                return kwargs
+            for _ in range(n):
+                x = f(**d)
+            return x
+
+        res, ex = self._run_with_optimizer(testfunc, TIER2_THRESHOLD)
+        self.assertEqual(res, {"a": 1, "b": 2})
+        uops = get_opnames(ex)
+
+        self.assertNotIn("_DICT_MERGE", uops)
+        self.assertIn("_MAKE_CALLARGS_A_TUPLE", uops)
+
     def test_list_extend(self):
         def testfunc(n):
             a = [1, 2, 3]

Modules/_testinternalcapi/test_cases.c.h

@@ -5630,20 +5630,11 @@ dummy_func(
             }
             PyObject *kwargs_o = PyStackRef_AsPyObjectBorrow(kwargs);
             if (kwargs_o != NULL && !PyDict_CheckExact(kwargs_o)) {
-                PyObject *dict_o = PyDict_New();
+                PyObject *dict_o = _PyEval_KwargsToDict(
+                    tstate, PyStackRef_AsPyObjectBorrow(func), kwargs_o);
                 if (dict_o == NULL) {
                     ERROR_NO_POP();
                 }
-                PyObject *dupkey = NULL;
-                int err = _PyDict_MergeUniq(dict_o, kwargs_o, &dupkey);
-                if (err < 0) {
-                    _PyEval_FormatKwargsError(tstate,
-                        PyStackRef_AsPyObjectBorrow(func), kwargs_o, dupkey);
-                    Py_XDECREF(dupkey);
-                    Py_DECREF(dict_o);
-                    ERROR_NO_POP();
-                }
-                assert(dupkey == NULL);
                 _PyStackRef temp = kwargs;
                 kwargs = PyStackRef_FromPyObjectSteal(dict_o);
                 PyStackRef_CLOSE(temp);

Python/bytecodes.c

@@ -3493,6 +3493,26 @@ _PyEval_FormatKwargsError(PyThreadState *tstate, PyObject *func, PyObject *kwarg
     }
 }
 
+/* Return a new exact dict with the items of the mapping 'kwargs',
+   raising the same TypeError as DICT_MERGE on failure. */
+PyObject *
+_PyEval_KwargsToDict(PyThreadState *tstate, PyObject *func, PyObject *kwargs)
+{
+    PyObject *dict = PyDict_New();
+    if (dict == NULL) {
+        return NULL;
+    }
+    PyObject *dupkey = NULL;
+    if (_PyDict_MergeUniq(dict, kwargs, &dupkey) < 0) {
+        _PyEval_FormatKwargsError(tstate, func, kwargs, dupkey);
+        Py_XDECREF(dupkey);
+        Py_DECREF(dict);
+        return NULL;
+    }
+    assert(dupkey == NULL);
+    return dict;
+}
+
 void
 _PyEval_FormatExcCheckArg(PyThreadState *tstate, PyObject *exc,
                           const char *format_str, PyObject *obj)