[PEP 696] Fix swapping TypeVars with defaults. (#19449)

- Fixes #19444. (added `testTypeVarDefaultsSwap`)
- Fixes #19362 (added `testTypeVarDefaultsSwap2`)

Changed the logic for recursion guards of `TypeVarType`: Instead of
always substituting `repl = repl.accept(self)`, and situationally
updating `repl.default = repl.default.accept(self)` if the result is a
`TypeVarType`, we now always update `repl.default =
repl.default.accept(self)` a priori and then only choose the expanded
`repl.accept(self)` if the result is a concrete type.

## New Tests

- `testTypeVarDefaultsSwap`
(https://mypy-play.net/?mypy=1.17.0&python=3.12&gist=d5a025a31ae3c8b9e2a36f4738aa1991)
- `testTypeVarDefaultsSwap2`
(https://mypy-play.net/?mypy=1.17.0&python=3.12&gist=d3ed42c82f7144967c97d846c4c041ef)

PS: closed earlier PRs #19447, since it contained debugging changes, and
#19448 because it didn't solve #19362.

Author: randolf-scholz

mypy/expandtype.py

@@ -251,9 +251,9 @@ 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)
+            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
 

test-data/unit/check-typevar-defaults.test

@@ -416,6 +416,38 @@ 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, 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