It appears that the following example may have unwanted undefined behavior in C++, although not in C:

  struct A { int x, y; };
  A passthrough(A a) { return a; }
  int main(void) {
   A a;
   a.x = 0;
   return passthrough(a).x;
  }

The default memberwise copying operation is not specified to be done in a way that is insensitive to indeterminate values.