Fix noncommutative joins with bounded TypeVars (#20345)

Fixes #20344

Author: hauntsaninja

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)
 

mypy/test/testtypes.py

@@ -1051,6 +1051,35 @@ 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.