gh-158072: Fix JIT crash with omitted default arguments

Lib/test/test_capi/test_opt.py

@@ -1812,6 +1812,37 @@ def testfunc(n):
         # __init__ resolution allows promotion of range to constant
         self.assertNotIn("_LOAD_GLOBAL_BUILTINS", uops)
 
+    # See https://github.com/python/cpython/issues/158072
+    def test_init_with_default_argument(self):
+        script_helper.assert_python_ok("-c", textwrap.dedent(f"""\
+            sentinel = object()
+
+            class WithDefault:
+                def __init__(self, value=sentinel):
+                    if value is not sentinel:
+                        pass
+
+            for _ in range({TIER2_THRESHOLD * 3}):
+                WithDefault()
+            """), PYTHON_JIT="1")
+
+    def test_init_with_changed_code_argcount(self):
+        class C:
+            def __init__(self, value):
+                self.value = False
+
+            def varargs_init(self, *args):
+                self.value = isinstance(args, tuple)
+
+        def testfunc(n):
+            for _ in range(n):
+                result = C(42).value
+            return result
+
+        self.assertFalse(testfunc(100))
+        C.__init__.__code__ = C.varargs_init.__code__
+        self.assertTrue(testfunc(TIER2_THRESHOLD * 3))
+
     def test_init_guards_removed(self):
         class MyPoint:
             def __init__(self, x, y):

Misc/NEWS.d/next/Core_and_Builtins/2026-09-24-18-44-40.gh-issue-158072.mHveWw.rst

@@ -0,0 +1 @@
+Fix a JIT crash when a class is instantiated with their default arguments

Python/optimizer_symbols.c

@@ -1388,17 +1388,27 @@ _Py_uop_frame_new(
     }
 
     // Initialize with the initial state of all local variables
+    arg_len = Py_MIN(arg_len, co->co_argcount);
     for (int i = 0; i < arg_len; i++) {
         frame->locals[i] = PyJitRef_RemoveUnique(args[i]);
     }
 
-    // If the args are known, then it's safe to just initialize
-    // every other non-set local to null symbol.
-    bool default_null = args != NULL;
+    // When args is available, missing parameters get defaults or *args/**kwargs.
+    // Other locals start as NULL. When args is NULL, treat all locals as unknown.
+    int parameter_count = co->co_argcount + co->co_kwonlyargcount;
+    parameter_count += (co->co_flags & CO_VARARGS) != 0;
+    parameter_count += (co->co_flags & CO_VARKEYWORDS) != 0;
 
     for (int i = arg_len; i < co->co_nlocalsplus; i++) {
-        JitOptRef local = default_null ? _Py_uop_sym_new_null(ctx) : _Py_uop_sym_new_unknown(ctx);
-        frame->locals[i] = local;
+        if (args == NULL) {
+            frame->locals[i] = _Py_uop_sym_new_unknown(ctx);
+        }
+        else if (i < parameter_count) {
+            frame->locals[i] = _Py_uop_sym_new_not_null(ctx);
+        }
+        else {
+            frame->locals[i] = _Py_uop_sym_new_null(ctx);
+        }
     }
 
     frame->callable = _Py_uop_sym_new_not_null(ctx);