Fix #11971 (#11972)

Allows PlaceholderNode objects before raising an error for the attempted
reuse of a member name inside an Enum.

Author: mrossinek

mypy/semanal.py

@@ -2871,7 +2871,7 @@ def analyze_name_lvalue(self,
         outer = self.is_global_or_nonlocal(name)
         if kind == MDEF and isinstance(self.type, TypeInfo) and self.type.is_enum:
             # Special case: we need to be sure that `Enum` keys are unique.
-            if existing:
+            if existing is not None and not isinstance(existing.node, PlaceholderNode):
                 self.fail('Attempted to reuse member name "{}" in Enum definition "{}"'.format(
                     name, self.type.name,
                 ), lvalue)

test-data/unit/check-enum.test

@@ -1810,4 +1810,4 @@ reveal_type(A.str.value)  # N: Revealed type is "Literal['foo']?"
 reveal_type(A.int.value)  # N: Revealed type is "Literal[1]?"
 reveal_type(A.bool.value)  # N: Revealed type is "Literal[False]?"
 reveal_type(A.tuple.value)  # N: Revealed type is "Tuple[Literal[1]?]"
-[builtins fixtures/tuple.pyi]
+[builtins fixtures/tuple.pyi]

test-data/unit/pythoneval.test

@@ -1573,3 +1573,15 @@ x: OrderedDict[str, str] = OrderedDict({})
 reveal_type(x)  # Revealed type is "collections.OrderedDict[builtins.str, builtins.int]"
 [out]
 _testTypingExtensionsOrderedDictAlias.py:3: note: Revealed type is "collections.OrderedDict[builtins.str, builtins.str]"
+
+[case testEnumValueWithPlaceholderNodeType]
+# https://github.com/python/mypy/issues/11971
+from enum import Enum
+from typing import Callable, Dict
+class Foo(Enum):
+    Bar: Foo = Callable[[str], None]
+    Baz: Foo = Callable[[Dict[str, "Missing"]], None]
+[out]
+_testEnumValueWithPlaceholderNodeType.py:5: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
+_testEnumValueWithPlaceholderNodeType.py:6: error: Incompatible types in assignment (expression has type "object", variable has type "Foo")
+_testEnumValueWithPlaceholderNodeType.py:6: error: Name "Missing" is not defined