Support NamedTuple-derived Enum (#2407)

Fix #1745.

Author: elazarg

mypy/subtypes.py

@@ -120,6 +120,8 @@ def visit_instance(self, left: Instance) -> bool:
         if left.type.fallback_to_any:
             return True
         right = self.right
+        if isinstance(right, TupleType) and right.fallback.type.is_enum:
+            return is_subtype(left, right.fallback)
         if isinstance(right, Instance):
             if left.type._promote and is_subtype(left.type._promote,
                                                  self.right,

test-data/unit/pythoneval-enum.test

@@ -118,3 +118,17 @@ takes_int(SomeExtIntEnum.x)
 def takes_some_ext_int_enum(s: SomeExtIntEnum):
     pass
 takes_some_ext_int_enum(SomeExtIntEnum.x)
+
+
+[case testNamedTupleEnum]
+from typing import NamedTuple
+from enum import Enum
+
+N = NamedTuple('N', [('bar', int)])
+
+class E(N, Enum):
+    X = N(1)
+
+def f(x: E) -> None: pass
+
+f(E.X)