Improve insert/emplace robustness to self insertion

[Jonathan Wakely]

On 04/07/16 15:55 +0100, Jonathan Wakely wrote:
>I'm getting nervous about the smart insertion trick to avoid making a
>copy, I have a devious testcase in mind which will break with that
>change. I'll share the testcase later today.

Here's a testcase which passes with libstdc++ but fails with libc++
because libc++ doesn't make a copy when inserting a T lvalue into
std::vector<T>:

#include <vector>
#include <memory>
#include <cassert>

struct T
{
  T(int v = 0) : value(v) { }
  T(const T& t);
  T& operator=(const T& t);
  void make_child() { child = std::make_unique<T>(value + 10); }
  std::unique_ptr<T> child;
  int value;
};

T::T(const T& t) : value(t.value)
{
  if (t.child)
    child.reset(new T(*t.child));
}

T& T::operator=(const T& t)
{
  value = t.value;
  if (t.child)
  {
    if (child)
      *child = *t.child;
    else
      child.reset(new T(*t.child));
  }
  else
    child.reset();
  return *this;
}

int main()
{
  std::vector<T> v;
  v.reserve(3);
  v.push_back(T(1));
  v.back().make_child();
  v.push_back(T(2));
  v.back().make_child();

  assert(v[1].child->value == 12);
  assert(v[1].child->child == nullptr);

  v.insert(v.begin(), *v[1].child);

  assert(v[0].value == 12);
  assert(v[0].child == nullptr);
}

The problem is that the object being inserted (*v[1].child) is not an
element of the vector, so the optimization assumes it is unchanged by
shuffling the existing elements. That assumption is wrong.

As far as I can see, this program is perfectly valid. It's slightly
contrived to prove a point, but it's not entirely unrealistic code.