added second test

Author: randolf-scholz

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:

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