TypeAliasType can leak memory if name is a str subclass

[JelleZijlstra]

Bug report

Bug description:

TypeAliasType's traverse implementation does not visit its name. This means that we can leak memory if name is passed an instance of a mutable str subclass.

from typing import TypeAliasType
import weakref
import gc

class S(str):
    pass

def still_alive(ref):
    obj = ref()
    if not isinstance(obj, S):
        return False
    T = obj.T
    if not isinstance(T, TypeAliasType):
        return False
    return T

name = S("name")
T = TypeAliasType(name, int)
name.T = T
ref = weakref.ref(name)
assert still_alive(ref)
del name
del T
gc.collect()
gc.collect()
assert still_alive(ref)

del ref().T
assert not still_alive(ref)

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec #140016
• [3.14] gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec (GH-140016) #140063
• [3.13] gh-140000: Traverse name attribute for TypeVar, TypeVarTuple, TypeAliasType, ParamSpec (GH-140016) #140073

[JelleZijlstra]

This also affects TypeVar, TypeVarTuple, and ParamSpec.

[efimov-mikhail]

Leak for all 4 types is confirmed on main, 3.14 and 3.13 branches. PR with fix is submitted.