Converting assignment operator is too generous

[anonymous]

This has been reported by Karol Nowacki. The following program compiles although it shoudl not.

#include <boost/optional.hpp>

enum E1 {e1};
enum E2 {e2};

int main()
{
  boost::optional<E2> o2(e2);
  boost::optional<E1> o1;
  o1 = o2; // SHOULD NOT WORK
}

It should not compile given that the following do not compile:

E1 v1 = e2; // FAILS
E1 v1_;
v1_ = e2;   // FAILS

[akrzemi1]

Duplicate of #11087