(I'm using gcc with -O2.)

This seems like a straightforward opportunity to elide the copy constructor, since there are no side-effects to accessing the value of a field in a bar's copy of a foo; but the copy constructor is called, since I get the output meep meep!.

#include <iostream>

struct foo {
  foo(): a(5) { }
  foo(const foo& f): a(f.a) { std::cout << "meep meep!\n"; }
  int a;
};

struct bar {
  foo F() const { return f; }
  foo f;
};

int main()
{
  bar b;
  int a = b.F().a;
  return 0;
}