From 47ebf8d623a28aab51dbabb4cf3bafa920cc585d Mon Sep 17 00:00:00 2001
From: hauntsaninja <hauntsaninja@gmail.com>
Date: Mon, 1 Dec 2025 12:20:56 -0800
Subject: [PATCH 1/3] Fix noncommutative joins with bounded TypeVars

Fixes #20344
---
 mypy/join.py | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/mypy/join.py b/mypy/join.py
index 0822ddbfd89a..a074fa522588 100644
--- a/mypy/join.py
+++ b/mypy/join.py
@@ -297,10 +297,15 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
         return self.s
 
     def visit_type_var(self, t: TypeVarType) -> ProperType:
-        if isinstance(self.s, TypeVarType) and self.s.id == t.id:
-            if self.s.upper_bound == t.upper_bound:
-                return self.s
-            return self.s.copy_modified(upper_bound=join_types(self.s.upper_bound, t.upper_bound))
+        if isinstance(self.s, TypeVarType):
+            if self.s.id == t.id:
+                if self.s.upper_bound == t.upper_bound:
+                    return self.s
+                return self.s.copy_modified(
+                    upper_bound=join_types(self.s.upper_bound, t.upper_bound)
+                )
+            # Fix non-commutative joins
+            return get_proper_type(join_types(self.s.upper_bound, t.upper_bound))
         else:
             return self.default(self.s)
 

From 6c6fa8f8d118165ecf224feb23af167f4c1695ed Mon Sep 17 00:00:00 2001
From: hauntsaninja <hauntsaninja@gmail.com>
Date: Mon, 1 Dec 2025 17:58:01 -0800
Subject: [PATCH 2/3] test

---
 mypy/test/testtypes.py | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/mypy/test/testtypes.py b/mypy/test/testtypes.py
index fc68d9aa6eac..fb5354fe4a14 100644
--- a/mypy/test/testtypes.py
+++ b/mypy/test/testtypes.py
@@ -1051,6 +1051,30 @@ 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)
 
+    def test_join_type_var_bounds(self) -> None:
+        tvar1 = TypeVarType(
+            "tvar1",
+            "tvar1",
+            TypeVarId(-100),
+            [],
+            self.fx.o,
+            AnyType(TypeOfAny.from_omitted_generics),
+            INVARIANT,
+        )
+        any_type = AnyType(TypeOfAny.special_form)
+        tvar2 = TypeVarType(
+            "tvar2",
+            "tvar2",
+            TypeVarId(-101),
+            [],
+            upper_bound=UnionType([TupleType([any_type], self.fx.std_tuple), TupleType([any_type, any_type], self.fx.std_tuple)]),
+            default=AnyType(TypeOfAny.from_omitted_generics),
+            variance=INVARIANT,
+        )
+
+        self.assert_join(tvar1, tvar2, self.fx.o)
+        self.assert_join(tvar2, tvar1, self.fx.o)
+
     # There are additional test cases in check-inference.test.
 
     # TODO: Function types + varargs and default args.

From 43a70bbf2844d1972c29711b835f3a118f2f563e Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 2 Dec 2025 02:09:26 +0000
Subject: [PATCH 3/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/test/testtypes.py | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mypy/test/testtypes.py b/mypy/test/testtypes.py
index fb5354fe4a14..f5f4c6797db2 100644
--- a/mypy/test/testtypes.py
+++ b/mypy/test/testtypes.py
@@ -1067,7 +1067,12 @@ def test_join_type_var_bounds(self) -> None:
             "tvar2",
             TypeVarId(-101),
             [],
-            upper_bound=UnionType([TupleType([any_type], self.fx.std_tuple), TupleType([any_type, any_type], self.fx.std_tuple)]),
+            upper_bound=UnionType(
+                [
+                    TupleType([any_type], self.fx.std_tuple),
+                    TupleType([any_type, any_type], self.fx.std_tuple),
+                ]
+            ),
             default=AnyType(TypeOfAny.from_omitted_generics),
             variance=INVARIANT,
         )