gh-96751: Remove dead code from `CALL_FUNCTION_EX` opcode (GH-96752)

Lib/test/test_extcall.py

@@ -382,6 +382,27 @@ Test a kwargs mapping with duplicated keys.
       ...
     TypeError: test.test_extcall.g() got multiple values for keyword argument 'x'
 
+Call with dict subtype:
+
+    >>> class MyDict(dict):
+    ...     pass
+
+    >>> def s1(**kwargs):
+    ...     return kwargs
+    >>> def s2(*args, **kwargs):
+    ...     return (args, kwargs)
+    >>> def s3(*, n, **kwargs):
+    ...     return (n, kwargs)
+
+    >>> md = MyDict({'a': 1, 'b': 2})
+    >>> assert s1(**md) == {'a': 1, 'b': 2}
+    >>> assert s2(*(1, 2), **md) == ((1, 2), {'a': 1, 'b': 2})
+    >>> assert s3(**MyDict({'n': 1, 'b': 2})) == (1, {'b': 2})
+    >>> s3(**md)
+    Traceback (most recent call last):
+      ...
+    TypeError: s3() missing 1 required keyword-only argument: 'n'
+
 Another helper function
 
     >>> def f2(*a, **b):

Misc/NEWS.d/next/Core and Builtins/2022-09-11-12-43-43.gh-issue-96751.anRT6a.rst

@@ -0,0 +1 @@
+Remove dead code from ``CALL_FUNCTION_EX`` opcode.

Python/ceval.c

@@ -4716,19 +4716,8 @@ handle_eval_breaker:
             PyObject *func, *callargs, *kwargs = NULL, *result;
             if (oparg & 0x01) {
                 kwargs = POP();
-                if (!PyDict_CheckExact(kwargs)) {
-                    PyObject *d = PyDict_New();
-                    if (d == NULL)
-                        goto error;
-                    if (_PyDict_MergeEx(d, kwargs, 2) < 0) {
-                        Py_DECREF(d);
-                        format_kwargs_error(tstate, SECOND(), kwargs);
-                        Py_DECREF(kwargs);
-                        goto error;
-                    }
-                    Py_DECREF(kwargs);
-                    kwargs = d;
-                }
+                // DICT_MERGE is called before this opcode if there are kwargs.
+                // It converts all dict subtypes in kwargs into regular dicts.
                 assert(PyDict_CheckExact(kwargs));
             }
             callargs = POP();