deferred column_property gets set to "expired" state on flush, unconditionally loads on unexpire

from sqlalchemy import *
from sqlalchemy.orm import *
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()


class A(Base):
    __tablename__ = 'a'
    id = Column(Integer, primary_key=True)
    a = Column(Integer)
    b = Column(Integer)

    aplusb = column_property(
        a + b, deferred=True
    )

e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)

s = Session(e)
s.add(A(a=1, b=2))
s.commit()

a1 = s.query(A).first()
assert 'aplusb' not in a1.__dict__
a1.b = 5
s.flush()
s.commit()
a1.b
assert 'aplusb' not in a1.__dict__

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