[class.copy.assign] Trivial assignment operators can skip member subobjects

[frederick-vs-ja]

Full name of submitter (unless configured in github; will be published with the issue): Jiang An

Reference (section label): [class.copy.assign]

Link to reflector thread (if any):

Issue description:

It's possible to assign between objects of a derived class via a trivial assignment operator of the base class. As a result, it's possible to synthesize a defaulted and trivial assignment operator that skip member subobjects from such a derived class.

It seems that no implementation supports such skipping per llvm/llvm-project#36386. All investigated implementations synthesize trivial assignment operators which don't skip member subobjects.

E.g.

struct B0 { int b0; };

struct B {
  B &operator=(const B &) = default;
  int x;
};

struct D : B0, B {
  using B::operator=;
private:
  D &operator=(const D &) && = default;
};

struct Q {
  Q &operator=(const Q &) = default;
  D d;
};

Q qa, qb;
int main() {
  qb.d.b0 = 42;
  qb = qa;
  if (qb.d.b0 != 42)
    abort(); // selected by all known implementations
}

It's questionable whether the skipping should even exist. Perhaps we should either make the synthesized assignment operator non-trivial or non-skipping.

Suggested resolution:

[frederick-vs-ja]

CC @hubert-reinterpretcast

[jensmaurer]

Which particular "synthesized assignment operator" are you talking about? Q::operator= ?

[frederick-vs-ja]

Which particular "synthesized assignment operator" are you talking about? Q::operator= ?

Yes. It's exactly Q::operator=.