fix(mypyc): skip __slots__ class variable emission

Keep __slots__ classified as a class variable for type checking, but do not attach it to native classes at runtime. Native classes use their own layout, and exposing inherited slots prevents compiled subclasses from being created.

Add a compiled inheritance regression that reproduces the CI failure.

Author: XuehaiPan

mypyc/irbuild/classdef.py

@@ -329,6 +329,9 @@ def add_attr(self, lvalue: NameExpr, stmt: AssignmentStmt) -> None:
         # Variable declaration with no body
         if isinstance(stmt.rvalue, TempNode):
             return
+        # Native classes use mypyc's own layout, so don't emit __slots__ at runtime.
+        if lvalue.name == "__slots__":
+            return
         # Only treat marked class variables as class variables.
         if not (is_class_var(lvalue) or stmt.is_final_def):
             return

mypyc/test-data/run-classes.test

@@ -1175,6 +1175,23 @@ assert f() == 10
 A.x = 200
 assert f() == 200
 
+[case testSlotsNotEmittedAsClassVar]
+class Base:
+    __slots__ = ("value",)
+    value: int
+
+class Child(Base):
+    pass
+
+def make_child() -> Child:
+    child = Child()
+    child.value = 1
+    return child
+[file driver.py]
+from native import make_child
+
+assert make_child().value == 1
+
 [case testClassVarDoesNotShadowMethodGlobal]
 from typing import ClassVar
 from testutil import assertRaises