Fix crash with custom ErrorCodes (#15327)

A class marked with `allow_interpreted_subclasses=True` implicitly
supports serialization while also allowing it to be subclassed by pure
Python code.

Fixes #15255


https://mypyc.readthedocs.io/en/latest/differences_from_python.html#pickling-and-copying-objects
https://mypyc.readthedocs.io/en/latest/native_classes.html#inheritance

mypy/errorcodes.py

@@ -14,7 +14,7 @@
 sub_code_map: dict[str, set[str]] = defaultdict(set)
 
 
-@mypyc_attr(serializable=True)
+@mypyc_attr(allow_interpreted_subclasses=True)
 class ErrorCode:
     def __init__(
         self,

test-data/unit/check-custom-plugin.test

@@ -1014,3 +1014,15 @@ reveal_type(MyClass.foo_staticmethod)  # N: Revealed type is "def (builtins.int)
 [file mypy.ini]
 \[mypy]
 plugins=<ROOT>/test-data/unit/plugins/add_classmethod.py
+
+[case testCustomErrorCodePlugin]
+# flags: --config-file tmp/mypy.ini  --show-error-codes
+def main() -> int:
+    return 2
+
+main()  # E: Custom error  [custom]
+reveal_type(1)  # N: Revealed type is "Literal[1]?"
+
+[file mypy.ini]
+\[mypy]
+plugins=<ROOT>/test-data/unit/plugins/custom_errorcode.py

test-data/unit/plugins/custom_errorcode.py

@@ -0,0 +1,20 @@
+from mypy.errorcodes import ErrorCode
+from mypy.plugin import Plugin
+from mypy.types import AnyType, TypeOfAny
+
+CUSTOM_ERROR = ErrorCode(code="custom", description="", category="Custom")
+
+
+class CustomErrorCodePlugin(Plugin):
+    def get_function_hook(self, fullname):
+        if fullname.endswith(".main"):
+            return self.emit_error
+        return None
+
+    def emit_error(self, ctx):
+        ctx.api.fail("Custom error", ctx.context, code=CUSTOM_ERROR)
+        return AnyType(TypeOfAny.from_error)
+
+
+def plugin(version):
+    return CustomErrorCodePlugin