From 2debb8652731550df9aec39e60d87cc40972a791 Mon Sep 17 00:00:00 2001
From: Shantanu Jain <hauntsaninja@gmail.com>
Date: Sun, 25 May 2025 19:19:16 -0700
Subject: [PATCH] Fix commutativity of join between TypeType and TypeVar

Fixes #18125
---
 mypy/join.py           | 2 ++
 mypy/test/testtypes.py | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/mypy/join.py b/mypy/join.py
index ac01d11d11d6..fcfc6cbaa0e7 100644
--- a/mypy/join.py
+++ b/mypy/join.py
@@ -635,6 +635,8 @@ def default(self, typ: Type) -> ProperType:
         typ = get_proper_type(typ)
         if isinstance(typ, Instance):
             return object_from_instance(typ)
+        elif isinstance(typ, TypeType):
+            return self.default(typ.item)
         elif isinstance(typ, UnboundType):
             return AnyType(TypeOfAny.special_form)
         elif isinstance(typ, TupleType):
diff --git a/mypy/test/testtypes.py b/mypy/test/testtypes.py
index a42519c64956..63d8840fa217 100644
--- a/mypy/test/testtypes.py
+++ b/mypy/test/testtypes.py
@@ -1064,6 +1064,10 @@ def test_variadic_tuple_joins(self) -> None:
             self.tuple(UnpackType(Instance(self.fx.std_tuplei, [self.fx.a])), self.fx.a),
         )
 
+    def test_join_type_type_type_var(self) -> None:
+        self.assert_join(self.fx.type_a, self.fx.t, self.fx.o)
+        self.assert_join(self.fx.t, self.fx.type_a, self.fx.o)
+
     # There are additional test cases in check-inference.test.
 
     # TODO: Function types + varargs and default args.