[mypyc] Fix interaction of Enum caching with _order_ (#8120)

Author: msullivan

mypyc/genops.py

@@ -1445,7 +1445,12 @@ def add_non_ext_class_attr(self, non_ext: NonExtClassInfo, lvalue: NameExpr,
             self.add_to_non_ext_dict(non_ext, lvalue.name, rvalue, stmt.line)
             # We cache enum attributes to speed up enum attribute lookup since they
             # are final.
-            if cdef.info.bases and cdef.info.bases[0].type.fullname == 'enum.Enum':
+            if (
+                cdef.info.bases
+                and cdef.info.bases[0].type.fullname == 'enum.Enum'
+                # Skip "_order_", since Enum will remove it
+                and lvalue.name != '_order_'
+            ):
                 attr_to_cache.append(lvalue)
 
     def setup_non_ext_dict(self, cdef: ClassDef, bases: Value) -> Value:

mypyc/test-data/run-classes.test

@@ -126,6 +126,7 @@ assert o.get(20) == 20
 from enum import Enum
 
 class TestEnum(Enum):
+    _order_ = "a b"
     a : int = 1
     b : int = 2
 
@@ -140,11 +141,14 @@ assert TestEnum.test() == 3
 # non-extension classes
 
 [file driver.py]
-from native import TestEnum
-assert TestEnum.a.name == 'a'
-assert TestEnum.a.value == 1
-assert TestEnum.b.name == 'b'
-assert TestEnum.b.value == 2
+import sys
+# "_order_" isn't supported in 3.5
+if sys.version_info[:2] > (3, 5):
+    from native import TestEnum
+    assert TestEnum.a.name == 'a'
+    assert TestEnum.a.value == 1
+    assert TestEnum.b.name == 'b'
+    assert TestEnum.b.value == 2
 
 [case testRunDataclass]
 from dataclasses import dataclass, field