[mypyc] Fix classes with __dict__ on 3.12 (#15471)

Got this working with a little trial and error.

Work on mypyc/mypyc#995.

mypyc/codegen/emitclass.py

@@ -284,8 +284,9 @@ def emit_line() -> None:
 
         fields["tp_members"] = members_name
         fields["tp_basicsize"] = f"{base_size} + 2*sizeof(PyObject *)"
-        fields["tp_dictoffset"] = base_size
-        fields["tp_weaklistoffset"] = weak_offset
+        if emitter.capi_version < (3, 12):
+            fields["tp_dictoffset"] = base_size
+            fields["tp_weaklistoffset"] = weak_offset
     else:
         fields["tp_basicsize"] = base_size
 
@@ -341,6 +342,8 @@ def emit_line() -> None:
             # This is just a placeholder to please CPython. It will be
             # overridden during setup.
             fields["tp_call"] = "PyVectorcall_Call"
+    if has_managed_dict(cl, emitter):
+        flags.append("Py_TPFLAGS_MANAGED_DICT")
     fields["tp_flags"] = " | ".join(flags)
 
     emitter.emit_line(f"static PyTypeObject {emitter.type_struct_name(cl)}_template_ = {{")
@@ -730,7 +733,9 @@ def generate_traverse_for_class(cl: ClassIR, func_name: str, emitter: Emitter) -
     for base in reversed(cl.base_mro):
         for attr, rtype in base.attributes.items():
             emitter.emit_gc_visit(f"self->{emitter.attr(attr)}", rtype)
-    if cl.has_dict:
+    if has_managed_dict(cl, emitter):
+        emitter.emit_line("_PyObject_VisitManagedDict((PyObject *)self, visit, arg);")
+    elif cl.has_dict:
         struct_name = cl.struct_name(emitter.names)
         # __dict__ lives right after the struct and __weakref__ lives right after that
         emitter.emit_gc_visit(
@@ -751,7 +756,9 @@ def generate_clear_for_class(cl: ClassIR, func_name: str, emitter: Emitter) -> N
     for base in reversed(cl.base_mro):
         for attr, rtype in base.attributes.items():
             emitter.emit_gc_clear(f"self->{emitter.attr(attr)}", rtype)
-    if cl.has_dict:
+    if has_managed_dict(cl, emitter):
+        emitter.emit_line("_PyObject_ClearManagedDict((PyObject *)self);")
+    elif cl.has_dict:
         struct_name = cl.struct_name(emitter.names)
         # __dict__ lives right after the struct and __weakref__ lives right after that
         emitter.emit_gc_clear(
@@ -1040,3 +1047,15 @@ def generate_property_setter(
         )
     emitter.emit_line("return 0;")
     emitter.emit_line("}")
+
+
+def has_managed_dict(cl: ClassIR, emitter: Emitter) -> bool:
+    """Should the class get the Py_TPFLAGS_MANAGED_DICT flag?"""
+    # On 3.11 and earlier the flag doesn't exist and we use
+    # tp_dictoffset instead.  If a class inherits from Exception, the
+    # flag conflicts with tp_dictoffset set in the base class.
+    return (
+        emitter.capi_version >= (3, 12)
+        and cl.has_dict
+        and cl.builtin_base != "PyBaseExceptionObject"
+    )