gh-119946: make Protocol compatible with Enum instantiation

Author: randolf-scholz

Lib/test/test_enum.py

@@ -2478,6 +2478,26 @@ class SomeEnum(Enum):
         globals()['T'] = T
         test_pickle_dump_load(self.assertIs, SomeEnum.first)
 
+    def test_protocol_mixin_as_enum_member_value(self):
+        class Example(typing.Protocol):
+            def method(self) -> None: ...
+
+        class Impl(Example):
+            def method(self) -> None: ...
+
+        class CompatEnumType(type(typing.Protocol), EnumType): ...
+
+        class ProtoEnum(Example, Enum, metaclass=CompatEnumType):
+            __qualname__ = 'ProtoEnum'  # needed for pickle protocol 4
+            impl = Impl()
+
+        self.assertIsInstance(ProtoEnum.impl.value, Impl)
+        globals()['Example'] = Example
+        globals()['Impl'] = Impl
+        globals()['ProtoEnum'] = ProtoEnum
+        ProtoEnum.__reduce_ex__ = enum.pickle_by_enum_name
+        test_pickle_dump_load(self.assertIs, ProtoEnum.impl)
+
     def test_duplicate_values_give_unique_enum_items(self):
         class AutoNumber(Enum):
             first = ()

Lib/typing.py

@@ -1882,7 +1882,11 @@ def _get_protocol_attrs(cls):
 def _no_init_or_replace_init(self, *args, **kwargs):
     cls = type(self)
 
-    if cls._is_protocol:
+    # Determine if this is a protocol or a concrete subclass.
+    # Note: not using cls._is_protocol here,
+    #  since this may be called before __init_subclass__ is resolved.
+    #  for example when subclassing enum.Enum.
+    if any(b is Protocol for b in cls.__bases__):
         raise TypeError('Protocols cannot be instantiated')
 
     # Already using a custom `__init__`. No need to calculate correct