Prevent crash when enum call is stored into class attribute

Author: sterliakov

mypy/semanal.py

@@ -3464,8 +3464,8 @@ def record_special_form_lvalue(self, s: AssignmentStmt) -> None:
     def analyze_enum_assign(self, s: AssignmentStmt) -> bool:
         """Check if s defines an Enum."""
         if isinstance(s.rvalue, CallExpr) and isinstance(s.rvalue.analyzed, EnumCallExpr):
-            # Already analyzed enum -- nothing to do here.
-            return True
+            # Already analyzed enum -- only recheck that it can be stored correctly.
+            return len(s.lvalues) == 1 and isinstance(s.lvalues[0], NameExpr)
         return self.enum_call_analyzer.process_enum_call(s, self.is_func_scope())
 
     def analyze_namedtuple_assign(self, s: AssignmentStmt) -> bool:

test-data/unit/check-enum.test

@@ -2512,3 +2512,15 @@ def list_vals(e: Type[T]) -> list[T]:
 
 reveal_type(list_vals(Choices))  # N: Revealed type is "builtins.list[__main__.Choices]"
 [builtins fixtures/enum.pyi]
+
+[case testEnumAsClassMemberNoCrash]
+# https://github.com/python/mypy/issues/18736
+from enum import Enum
+
+class Base:
+    def __init__(self, namespace: tuple[str, ...]) -> None:
+        # Not a bug: trigger defer
+        names = [name for name in namespace if fail]  # E: Name "fail" is not defined
+        self.o = Enum("o", names)  # E: Enum type as attribute is not supported \
+                                   # E: Second argument of Enum() must be string, tuple, list or dict literal for mypy to determine Enum members
+[builtins fixtures/tuple.pyi]