stubtest: Add support for setting enum members to "..." (#16807)

Unblock python/typeshed#11299

Author: JelleZijlstra

mypy/stubtest.py

@@ -1093,6 +1093,13 @@ def verify_var(
             runtime_type = get_mypy_type_of_runtime_value(runtime.value)
             if runtime_type is not None and is_subtype_helper(runtime_type, stub.type):
                 should_error = False
+            # We always allow setting the stub value to ...
+            proper_type = mypy.types.get_proper_type(stub.type)
+            if (
+                isinstance(proper_type, mypy.types.Instance)
+                and proper_type.type.fullname == "builtins.ellipsis"
+            ):
+                should_error = False
 
         if should_error:
             yield Error(

mypy/test/teststubtest.py

@@ -1139,6 +1139,25 @@ def baz(x=Flags3(0)): pass
             """,
             error=None,
         )
+        yield Case(
+            runtime="""
+            import enum
+            class SomeObject: ...
+
+            class WeirdEnum(enum.Enum):
+                a = SomeObject()
+                b = SomeObject()
+            """,
+            stub="""
+            import enum
+            class SomeObject: ...
+            class WeirdEnum(enum.Enum):
+                _value_: SomeObject
+                a = ...
+                b = ...
+            """,
+            error=None,
+        )
         yield Case(
             stub="""
             class Flags4(enum.Flag):