sum() deallocates float subclass instances as exact floats in the complex fast path

Bug report

Bug description:

Bug report

Description

In builtin_sum_impl() (Python/bltinmodule.c), the complex-number fast
path tests items with PyFloat_Check() but releases them with
_Py_DECREF_SPECIALIZED(item, _PyFloat_ExactDealloc).

PyFloat_Check() also matches subclasses of float, whereas
_PyFloat_ExactDealloc is only valid for exact float objects. When a
float subclass instance reaches refcount 0 through this path, it is
deallocated as an exact float, bypassing its real tp_dealloc
(instance __dict__, weakref list, and GC bookkeeping). This can leak
memory and leave the cyclic GC in an inconsistent state.

The adjacent float fast path in the same function already guards the
identical specialized dealloc with PyFloat_CheckExact(); only the
complex branch uses the broader PyFloat_Check().

Reproducer

class F(float):
    pass

# complex start value selects the complex fast path
sum([F(1.0), F(2.0)], 0j)

Proposed fix

In the complex branch, use PyFloat_CheckExact(item) instead of
PyFloat_Check(item), so subclass instances fall through to the generic
PyNumber_Add path with a normal Py_DECREF. Numeric behavior is
unchanged.

Environment

• CPython: main branch (please confirm the line is still present)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-150869
• gh-151063

從 Python/bltinmodule.c 中的 builtin_sum_impl() 開始，將 complex 的快速路徑與相鄰的 float 快速路徑進行比較。驗證針對 float 子類別的專用釋放檢查，並確認 reproducer 完成時子類別沒有被錯誤地釋放；關聯的 PR gh-150869 和 gh-151063 表明相關工作可能已經在進行中。