[PEP 696] Assigning to type variables with defaults gives unexpected results

[randolf-scholz]

Code sample in pyright playground mypy playground

class Foo[X, Y = object]:
    def test(self) -> None:
        reveal_type(Foo[Y, X]())  # Foo[X`1, X`1]

class Bar[X = object, Y = object]:
    def test(self) -> None:
        reveal_type(Bar[Y, X]())  # Bar[Y`2 = builtins.object, Y`2 = builtins.object]

Expected:

• Foo[Y@Foo, X@Foo]
• Bar[Y@Bar, X@Bar]

Related issue: #19362

[randolf-scholz]

It goes wrong here:

mypy/mypy/expandtype.py

Lines 245 to 252 in c66417d

	if isinstance(repl, TypeVarType) and repl.has_default():
	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

[randolf-scholz]

Would this work?

if isinstance(repl, TypeVarType) and repl.has_default():
    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
return repl