From 510a89d584cce0932743d29f4614b2af8a7bf7a2 Mon Sep 17 00:00:00 2001
From: Randolf Scholz <randolf.scholz@gmail.com>
Date: Tue, 15 Jul 2025 14:08:45 +0200
Subject: [PATCH 1/2] initial commit + revert debugging

---
 mypy/expandtype.py                         | 12 ++++++++----
 test-data/unit/check-typevar-defaults.test | 18 ++++++++++++++++++
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/mypy/expandtype.py b/mypy/expandtype.py
index d27105f48ed3..100dc94c8639 100644
--- a/mypy/expandtype.py
+++ b/mypy/expandtype.py
@@ -246,10 +246,14 @@ def visit_type_var(self, t: TypeVarType) -> Type:
             if (tvar_id := repl.id) in self.recursive_tvar_guard:
                 return self.recursive_tvar_guard[tvar_id] or repl
             self.recursive_tvar_guard[tvar_id] = None
-            repl = repl.accept(self)
-            if isinstance(repl, TypeVarType):
-                repl.default = repl.default.accept(self)
-            self.recursive_tvar_guard[tvar_id] = repl
+            expanded = repl.accept(self)
+
+            if isinstance(expanded, TypeVarType):
+                expanded.default = expanded.default.accept(self)
+            else:
+                repl = expanded
+
+            self.recursive_tvar_guard[tvar_id] = expanded
         return repl
 
     def visit_param_spec(self, t: ParamSpecType) -> Type:
diff --git a/test-data/unit/check-typevar-defaults.test b/test-data/unit/check-typevar-defaults.test
index 22270e17787e..b90a3d5789d1 100644
--- a/test-data/unit/check-typevar-defaults.test
+++ b/test-data/unit/check-typevar-defaults.test
@@ -416,6 +416,24 @@ def func_c4(
     reveal_type(m)  # N: Revealed type is "__main__.ClassC4[builtins.int, builtins.float]"
 [builtins fixtures/tuple.pyi]
 
+[case testTypeVarDefaultsSwap]
+from typing import TypeVar, Generic
+
+T = TypeVar("T")
+X = TypeVar("X", default=object)
+Y = TypeVar("Y", default=object)
+
+
+class Foo(Generic[T, Y]):
+    def test(self) -> None:
+        reveal_type( Foo[Y, T]() )  # N: Revealed type is "__main__.Foo[Y`2 = builtins.object, T`1]"
+
+
+class Bar(Generic[X, Y]):
+    def test(self) -> None:
+        reveal_type( Bar[Y, X]() )  # N: Revealed type is "__main__.Bar[Y`2 = builtins.object, Y`2 = builtins.object]"
+
+
 [case testTypeVarDefaultsClassRecursive1]
 # flags: --disallow-any-generics
 from typing import Generic, TypeVar, List

From 4b34af11984a56785161c6295934e683a838f72e Mon Sep 17 00:00:00 2001
From: Randolf Scholz <randolf.scholz@gmail.com>
Date: Tue, 15 Jul 2025 17:38:48 +0200
Subject: [PATCH 2/2] added second test

---
 mypy/expandtype.py                         | 12 ++++--------
 test-data/unit/check-typevar-defaults.test | 16 +++++++++++++++-
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/mypy/expandtype.py b/mypy/expandtype.py
index 100dc94c8639..129818b658cc 100644
--- a/mypy/expandtype.py
+++ b/mypy/expandtype.py
@@ -246,14 +246,10 @@ def visit_type_var(self, t: TypeVarType) -> Type:
             if (tvar_id := repl.id) in self.recursive_tvar_guard:
                 return self.recursive_tvar_guard[tvar_id] or repl
             self.recursive_tvar_guard[tvar_id] = None
-            expanded = repl.accept(self)
-
-            if isinstance(expanded, TypeVarType):
-                expanded.default = expanded.default.accept(self)
-            else:
-                repl = expanded
-
-            self.recursive_tvar_guard[tvar_id] = expanded
+            repl.default = repl.default.accept(self)
+            expanded = repl.accept(self)  # Note: `expanded is repl` may be true.
+            repl = repl if isinstance(expanded, TypeVarType) else expanded
+            self.recursive_tvar_guard[tvar_id] = repl
         return repl
 
     def visit_param_spec(self, t: ParamSpecType) -> Type:
diff --git a/test-data/unit/check-typevar-defaults.test b/test-data/unit/check-typevar-defaults.test
index b90a3d5789d1..103c0e782797 100644
--- a/test-data/unit/check-typevar-defaults.test
+++ b/test-data/unit/check-typevar-defaults.test
@@ -431,9 +431,23 @@ class Foo(Generic[T, Y]):
 
 class Bar(Generic[X, Y]):
     def test(self) -> None:
-        reveal_type( Bar[Y, X]() )  # N: Revealed type is "__main__.Bar[Y`2 = builtins.object, Y`2 = builtins.object]"
+        reveal_type( Bar[Y, X]() )  # N: Revealed type is "__main__.Bar[Y`2 = builtins.object, X`1 = builtins.object]"
 
 
+[case testTypeVarDefaultsSwap2]
+from typing import TypeVar, Generic
+
+X = TypeVar("X", default=object)
+Y = TypeVar("Y", default=object)
+U = TypeVar("U", default=object)
+V = TypeVar("V", default=object)
+
+class Transform(Generic[X, Y]):
+    def invert(self) -> "Transform[Y, X]": ...
+
+class Foo(Transform[U, V], Generic[U, V]):
+    def invert(self) -> "Foo[V, U]": ...
+
 [case testTypeVarDefaultsClassRecursive1]
 # flags: --disallow-any-generics
 from typing import Generic, TypeVar, List